• The Use CP volume check box
If you skip step 3), the sound will be played at the same level as the one of the Beep, which then acts as a "master" control. I called this the Control Panel volume or CP volume for short, because under system 6.0.x you can also set it with the General Control Panel device.
Please note that if you do adjust the volume with the slider in SndControl, the check in the Use CP volume box will disappear, indicating that this particular sound will be played at its own, independent volume.
To force the sound to be played at whatever volume is set for the Beep, simply re-check the Use CP volume box.
Also note that whenever the Beep or CP volume is set to zero, SndControl will respect your wish and your Mac will remain totally silent, no matter what volume the various sounds are set to play at. This is especially convenient late into the night or when your boss is around...

• The Randomize check box
Having the same sound played every time a given event occurs can and often does become very boring or even irritating at times.
SndControl provides you with an easy way to fix that: Any event can be assigned a randomly selected sound by simply checking the Randomize check box. When you do, the list of sounds will be replaced by a list of sound suitcases present in your System Folder or in a folder called Sounds (within the System Folder).
If you press the option key as you check Randomize, you'll get a dialog allowing you to select a folder ANYWHERE on your hard disk. The location of this folder will be stored in the prefs file and remembered. Any sound suitcases you place in that folder will appear in the list when you next check the Randomize box.
NB: Under system 6, you'll have to select a suitcase in a folder and click Open to select the folder.

Selecting a suitcase's name tells SndControl to pick a sound at random from those contained in that suitcase. This way, you can create thematic suitcases to be assigned to a particular type of event (such as one containing 23 different kinds of laughter for when you insert a bad (i.e. unformatted) disk, or ten different appreciative phrases to thank you for inserting a good disk and so on).

Selecting the line at the top of the list means that the sound will be picked at random amongst those available in the system file (plus those in any suitcases opened with Suitcase, Master Juggler or CarpetBag).

To revert to having a fixed sound played, simply uncheck the Randomize box.

• The Keep In RAM check box
By default the sound to be played for a given event will be fetched from disk when needed and purged from internal RAM memory when it's finished playing.
This works fine most of the time and avoids using up memory for nothing. The penalty for doing so is that there will be a noticeable delay as the sound is read in from disk. If the delay bothers you, check this box and the sound will be kept in RAM. 

Please note that if there is insufficient memory any time a sound needs to be loaded from disk, the Mac will simply remain silent.

When Randomize is on, the Keep in RAM check box will be unselectable, since it makes no sense to try and keep a randomly selected sound in memory (only to be discarded when the next random selection is made). Setting the key strokes to play random sounds can be fun, but you'll have to put up with the time it takes to fetch each new sound from disk, I'm afraid!

• The Show Icon check box
This is normally checked. Uncheck it if you do not wish to see the SndControl icon displayed at the bottom of the screen during startup.

• The Don't load check box
This is normally not checked. Put a check in it if you want to disable SndControl from the next time you restart, yet leave it in the System Folder.

• The Size information
The number displayed is the size in bytes of the sound you have selected for an event. 

• A word on IdleTalk (patent pending! grin, grin!)
This feature was suggested to me by my son Guy, who was trying to find new uses for the sounds he sampled over the 1989 Christmas holidays. 
What it does, is play the sound of your choice whenever a predefined period of inactivity has elapsed.
I currently have a 772K sound suitcase filled with 36 very appropriate things to say, such as : "Don't pick your nose!" or "Oh my God, he's gone!" or "Type something, will you, we're paying for this" and so on (it fits on one disk and I'll send it to you if you add an extra $3 when you register). Having these sounds picked at random can be very hilarious, as they often seem very appropriate to what you are doing (or not doing as the case may be) at the time they're uttered.
By default, IdleTalk is off. Turn the IdleTalk radio button on and you'll see two little arrows appear. Click the up/down arrow to increase/decrease the amount of idle time (in minutes) that you want your Mac to wait for before throwing idle noises at you. Clicking the down arrow when the value is 1 (one) will turn IdleTalk off.
Just in case you're wondering, IdleTalk often utters abuse at me right in the middle of a Navigator connection to CompuServe or bang in the middle of DiskExpress II's optimization process and it causes no harm (on my own Mac, anyway).
Someone recently suggested using IdleTalk to put off burglars by recording voices and dog barks to play at random!!!

• A word on PhraseTalk
This feature was requested a long time ago by a user who wanted to reduce the number of Extensions in his system folder yet not give up the fun function of one called "I'm Batman" by Steve Riggins. It allows you to have a sound played whenever a given word or phrase is typed. You may change the phrase and its associated sound at any time, without the need to restart your Mac.

• The Copy Done sound
This is actually quite useful, since it will play a sound when the Finder has finished duplicating files or copying them from disk to disk.
If your System is not in English, you may change the word "Copy" to reflect the title of that window (under 7.0.x) in your own language. Also note that under System 6.0.x setting the title to "D" does the trick. (However, some 6.0.x users reported this does not work for them and might require a bit of ResEdit surgery.)

• The Alarm sound will play whenever the alarm you set with Apple's Alarm Clock DA goes off.

• Please note that it is "normal" for the Zoom in/out sounds sometimes to remain silent under 7.0.x's Finder. (The Finder sometimes doesn't call the ZoomWindow trap.)

• In Filemaker Pro, Excel 4 and possibly other programs, don't be surprised to hear the CloseWindow or ZoomWindow sounds when you least expect them. For example, you'll hear a CloseWindow sound when a pop up list disappears in Filemaker and the zoomwindow sound every time you open a window in Excel or Think C.
C'est la vie…

• A word on LaunchSnd
If you want a sound to be played when a certain application is launched, use Sound Mover and:
1) copy the sound into the application itself (hold down the option key as you click Open in Sound Mover to see a list of all files on your disk, including applications);
2) rename the sound you placed in the application to "LaunchSnd".
That's it. The sound will be played every time you launch that application (unless you disable SndControl, that is). To silence it, either hold down the option key while you launch, turn the volume down to zero, rename the sound to something different from LaunchSnd or remove it from the application altogether.
Credit is due to Dennis Chesters for suggesting this feature.
Note that if large sounds fail to play under System 6.0.x's MultiFinder, you may have to increase the amount of memory allotted to the application by an amount roughly equal to the size of the sound.

• A word on the special "snd" suitcase
In order to be available to SndControl, 'snd ' resources must reside in an open resource file. This is the same requirement that applies to Fonts and DAs. Unless you own either Suitcase, MJ or CB, the only resource file that is always open is the actual System File. This is where you'd normally have to install Fonts and DAs, using Font/DA Mover, and sounds, using Sound Mover, ResEdit or similar utility (or simply dropping them there under System 7.0.x). In order to avoid forcing you to alter the System File by installing snd's directly into it, SndControl will automatically open a resource file named "snd" if you place it in the System Folder. You may create this file with ResEdit and fill it with any 'snd 's you want to make available, but it's far easier to create it and edit it using Sound Mover, the companion to SndControl in the Sound Manager Package.

• A word on the Trash sounds
These sounds are played when the Trash Can icon first becomes either fat or thin. Occasionally, this may produce surprising results, such as being told "Erased from existence" when in fact you've emptied the can by pulling the document(s) it contained out to safety. Rest assured that nothing has been erased. It's just that computers (or rather, us human programmers) are occasionally dumber than you may think <grin>.
Also note that if you customize your trash can (e.g. to show a toilet seat or a black hole or something), SndControl will not play the sounds, since it depends on the shape of the original icon.

• About the Chimes
You can select a sound to be played on the hour, quarter past, half past and quarter to. You may also assign a sound to count the number of hours on the hour. Note that pressing the option key will allow the currently playing sound to finish, but stop all further sounds until you release it. Useful if you know it's midday and cannot be bothered listening to twelve dongs!

• Silence is golden
To avoid having the sound associated with a particular event played, simply set its volume to zero or click on the "No sound" line at the top of the list of available sounds. To silence the Mac completely, irrespective of the various sound settings, set the volume to zero in SndControl when the Beep radio button is on.
To temporarily silence the Mac, hold down the option key just before and during the event that would trigger the sound (e.g. holding option down as you Empty Trash will prevent that sound from being played).
Pressing the Esc key will stop the currently playing sound.
To disable SndControl completely, you will need to restart the Mac. Before restarting, check the Don't load box in the cdev or remove the SndControl file from the System Folder altogether. Or you may hold down the shift key while restarting, just prior to when you expect its icon to appear. You'll know SndControl hasn't loaded if its icon has a cross on it. 
To avoid having the startup sound played, press the Caps lock key during the startup (booting) process. SndControl will remind you of your decision to silence it by displaying an icon that says Shh and Caps!
NB: The icons mentioned above will only show if you leave the Show icon box checked in the cdev.