If you launch Supermodel from the command line or using a batch file you can change the in game music, SFX and overall sound balance using the following commands:
-music-volume=100 -sound-volume=90 -balance=100
Just change the number after the equals sign to whatever you finds works best for you - you'll probably need to experiment a bit to get the balance right, it'll be affected by what type of speaker set-up your PC's got. I have different music to SFX ratios for different games. I thiiiink (been a long time since I've messed with any of these setting) 100 is the default, so you can go higher or lower. The balance part relates to games that had speakers behind and in front of you that played different things, and can be useful if your music to SFX to in-game speech balance needs further fine tuning. Again, there's no standardised setting, you'll need to experiment with what sounds right for you.