Make each mp3 folder have the same average volume

Here are the steps to follow if you have folders of mp3s, and
A) you want to make each whole folder have a chosen overall volume, but
B) you want the volume differences between the individual mp3s inside each folder to remain the same.

(MP3Gain automatically assumes that mp3s in different folders are in different "albums")

Step 1 Click the Add Folder button
Step 2 Navigate to where the folder that you want to convert is and select it


Repeat as needed if you have more than one folder you want to normalize
Step 3 Set your Target Volume in Decibels (dB) in the Target "Normal" Volume field.

Step 4 Click the Album Analysis button

MP3Gain will analyze all of the files and report on each song's volume level, whether any clipping occurred, and other details including the combined album volume levels.
If you are not using tags, then make note of the mp3's original volume level (the value under the Volume column) if you want to modify the volume setting back to its original value. You can also check the Analysis log if you have it enabled for these details.

Step 5 Click the Album Gain button

This will adjust all the mp3s in the album so that the overall volume of each album is as close to the Target "Normal" Volume as possible. The volume differences between the mp3s on the album will stay the same.

Step 6 Listen to your new mp3s. If they sound too loud or quiet, adjust the Target Volume setting and click Album Gain again.

NOTE: You can skip step 4 if you don't care what the original album volume was. If you press the Album Gain button before analysis has been done, it will automatically do analysis first. (Otherwise, how would it know what gain adjustment to make?)