It changes the % probability of the actions you make to succeed.I always turn off music in porn games and now I've discovered this game has "map mood" mechanic and music is the only(?) indicator of it. What an odd design. They(devs) could've put a warning on bgm mute button. Also, does anyone know how this mechanic works(what changes the "map mood")?
Even with music turned off you do not really miss much, you still see the % numbers anyway.