I talked about the sound problem with our musician and FMOD admin. He made a new version of the project and now it seems that - if the sound kindly joins us - the music and SFX volume values are reacting to the slider changes!

It worked in the Editor, in the Build and Run environment and - on my second attempt of starting the game - on the itch website, too.

Good luck and lots of sound to you!

~ fingers crossed ~