When starting Minecraft, the mod reverts the master volume to the default value (at least for unfocused/invisible). The config shows the correct values though.
v3.5.0-beta.1 MC 1.21

I hadn't tested on other volumes and just the master volume. I'll give this a shot and see! Thanks!!

Well I think I've fixed the issue you've experienced, if I misunderstood please let me know.

Interesting! I can reproduce this when setting the master volume for the unfocused state to 100%, but not to any other value, is that also the case for you or does it happen for any percentage?

Seems to be working!

Awesome :)