This is a little client-side mod that makes sound more interesting. Right now it adds reverb to caves and mutes sounds when you're underwater and when the sound source is behind a wall.
You are most definitely allowed to use this in modpacks!
I probably won't check this page here very often, so if you want to give a comment or report a bug, it's better if you do so on the minecraft forum page.
And here's a short video I made explaining the features: