• Disclaimers and known incompatibilities
Sorry, NO GUARANTEES:
I'm afraid I am in no position to guarantee that SndControl will work as advertized on your particular system: there are just too many hardware/software combinations out there for anyone to test them all.
For what it's worth, the program was thoroughly tested both by myself and by a fair number of my friends in the European Community Computer Club (ECCC), whom I'd collectively like to thank for allowing very early beta versions of SndControl to enter their System Folders. It's now been widely distributed for well over two years and I've heard of no major disasters due to its use, but I'd be either very arrogant or downright dishonest if I assured you that you won't have any problems at all. What I can assure you of is that I'll look very closely at all well documented bug reports and do my best to fix them.
If you can pinpoint a problem down to the use of SndControl, I'd really like to have details from you (what Mac model, how much memory, system version, what INITs and cdevs, what application(s) you were using, was it under MultiFinder, etc. Use MugShot, InitScope or similar utility to describe your system configuration, if you have it.). 

If you find any programs under which you think SndControl should be disabled, use the Exclude button and Add them to the list. The next time you run an application from this list, SndControl will be inactive. I recommend that you disable SndControl under Hypercard, •Sound Mover•, SoundEdit and any other programs that make heavy use of sound.

• SndControl and other INITs
Make sure SndControl loads after Suitcase if you've selected a startup sound that is contained in a suitcase compressed with Font & Sound Valet™. To accomplish this, simply rename one or the other in a way that changes their alphabetical order (e.g. aSuitcase or zSndControl). Under system 7, Suitcase will always load before SndControl, since Extensions load before Control Panels.

Registered User Michael Ross says that "if SmartQuotes [INIT 1.2] loads BEFORE SndControl, it will pretty randomly bomb a Mac SE (especially in CE Software's Disktop)." So make sure it loads AFTER SndControl by renaming it zSmartQuotes.

Beta tester and ECCC member André "if it's an INIT, I have it" Brugmans reports that although SndControl seems to live peacefully with a whole bunch of INITs on both his SE/30 and PB140, it must not be renamed to load before CEToolbox or Restart and Shutdown will be silent.

MultiFinder note (for System 6.0.x users only):
When using MultiFinder, each application (including the Finder itself) is allotted a certain amount of memory, called a "partition". If you get messages telling you that the application doesn't have enough memory to do this or that (e.g. Finder can't open any more windows, or something), you can increase the amount of memory assigned to that application.
For example, Finder comes preset with a 160 K partition size. I have increased that number to 220 K to give it a bit more breathing space and to allow even large sounds to be read in from disk and played. To do this, you click on the application once to select it, then you do a Get Info and change the number that appears in the lower right of the Get Info window. (Refer to your Macintosh system manuals, if in doubt.) 
If you notice that some sounds are not played even though they should be, under a particular application, that's a sure sign that you should increase the memory partition allotted to it. If you don't, there'll be no damage done, but larger sounds will simply not be loaded and played.
(Note that the above does not apply to System 7.0, since the system heap will automatically expand and shrink as needed.)