I currently have this mod enabled, with Master volume at 15%, Music at 10%, blocks at 20% etc. Whenever I alt tab and the mod kicks in, it reduces my fps as it should, and reduces that volume even lower by the multiplier set in the ModMenu > DynamicFPS settings. However, when I click back in the game, it just blasts my volume all the way back to 100%.

It doesn't change my in game settings, the sliders don't move. All I need to do is go back in the Music & Sound tab and click Done and it goes back to my usual settings. But I shouldn't need to do that, the mod should remove the multiplier and go back to my set volume, not 100%,

You're running and outdated version of the mod, this was fixed in version 3.3.1.

Also:

and reduces that volume even lower by the multiplier set in the ModMenu > DynamicFPS settings

The volume multiplier is called a multiplier and not just "volume" because it multiplies your current volume by this value. If you have music set to 50% and apply a 50% multiplier to music you will end up with 25% volume while tabbed out.

Thanks for the reply!

Yes I am indeed running an outdated version, as I haven't touched Minecraft at all for a while and forgot how to work my way around PrismLauncher. I updated it to 3.3.3, but the required Fabric launcher 15.6 broke the rest of my modpack. Manually selected 3.3.1 now, works wonders.

and reduces that volume even lower by the multiplier set in the ModMenu > DynamicFPS settings

And yes, I understand the multiplier thing. Sorry if I was not clear, I just wanted to describe that the multiplier was working as intended. English is not my first language, maybe I could have worded it better.

No worries!