I'm on Windows, so I can just use the Windows sound controls to turn them down. But it would be nice if the volume sliders worked.
Of course, I understand that :) So, as I mentioned, I'm out of the country, so I'm not in a situation to change the source code, but I did make a quick fix for you:
Step one: Download this file from Dropbox (download button is on the top right corner)
Link: https://www.dropbox.com/scl/fi/fsfnlpb1z3fmojcc5jj4b/options.rpy?rlkey=n92zn9zg0...)
Step two: Move this file to the game folder (...\ValentinesRain-1.0-pc\game). When it asks to replace the file, click yes.
This will attach all the looping channels to the music slider in the preference screen, so you will be able to control the sound that way, in game.
Hopefully this fixes it! Once I'm back home, I'll try to look into updating the game (there's a few other fixes I have in mind for it). Let me know if you have other questions =)
Drac