Sound Filters Mod
This is a client-side mod for 1.6.4. Right now it adds reverb to caves and mutes sounds when you’re underwater and when the sound source is behind a wall.
Tmtravlr do plan on updating this to 1.7, and adding an API so other people can easily create their own sound filter mods.
Tmtravlr 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.
Download Sound Filters Mod