Changing the menu sounds using a custom sounds.json does not change the menu sounds. I changed it so that the menu plays calm1, but it doesnt work... I will attach the sounds.json file later if necessary.

Edit:
I added the "replace" directive, and it fixed the problem! Sorry for wasting your time. If anybody else is having trouble getting a custom sounds.json file to work, please check out MC-48826, it might be that you need to add a "replace" directive to get your sound events to override default ones.