This mod is awesome. It's pretty old, and only updated to 1.5.0, but seriously. It adds dynamic sound, basically. Echoes in caves. The nether is spooky as hell with echoing ghast crys instead of annoying ARRCG right in your ears. No zombie grunts (or any sound) through thick walls. Jukeboxes will be faint in the next room over. It's so awesome...
Check it out.
http://www.minecraftforum.net/topic/355416-150sspsmp-bettersound-mod-bsm-alpha-v0016/
Check it out.
http://www.minecraftforum.net/topic/355416-150sspsmp-bettersound-mod-bsm-alpha-v0016/