Thought I fixed this, I did not. It works (somewhat) in 1.16.4, but not 1.16.3.
The muffling through walls and water works, but the reverb in caves does not. That's a big part of the experience for me. It works perfectly for my friend, and we are using all the same versions for forge and minecraft as well as the same mods. The only other mod we are using is optifine.

Can you please send the debug.log inside your .minecraft/logs folder ?

Using Forge 1.16.3 34.1.0
Same issue.
No other mods.
debug.log

The reverb does work, in very large stone boxes, like 40x40x40
I think it has something to do with how close you are to a wall.

Did some config testing and seems like Global Echo Multiplier isn't working at all.
Global Block Reflectance gave some reverb, but when I compare the echo of 1.12 with 1.16 echo.. the 1.12 echo has something that 1.16 doesn't.

Ah, I don't think i have it anymore. I haven't used this mod since then for version 1.16 because there aren't many other mods to pair it with.