1- Download MP3Gain.
2 - Install the software and launch it.
3 - Click on "Add Folder" on the upper left corner.
4 - Add the music folder of the game (C:\Users\PC-NAME\Documents\Rockstar Games\GTA V\User Music).
5 - Now set the "Target 'Normal' Volume" between 95 to 100 dB (Higher is louder).
6 - Select all of the files and click "Track Gain".
7 - Let the software do it's job and launch the game.
8 - Re-scan the files and you all good.
NOTE: if the volume of the songs is still too low, set the dB beyond 100 (Not recommended).