I would like to be able to add a sound mixer.

[HeiwaTeto]

Since there are only a few types of sound mixers in Minecraft, we find ourselves in the situation of "if I turn off this sound, that sound also disappears, which is frustrating," so we would like to see better support so that we can create our own sound mixers.

Edited August 3 by HeiwaTeto

[LexManos]

Thats not something Forge will add, but there are events for when sounds are played, and pretty sure you have the ability to adjust the volumes in that event.