Loads of resource packs exist with custom sounds. However, one thing that is generally overlooked is actually modding sounds themselves. More specifically, modding how sounds work inside the game.
SoundEther has put together exactly that, with Sound Physics: it overhauls Minecraft's sound engine to simulate reverberation, sound occlusion/absorption, and physically based sound attenuation. Ever wanted to tell where a creeper was coming from based solely on the noises it makes? Sound Physics allows that! Want caves to sound appropriately echo-y and grand? It does that! The distance a sound travels, and how much clutter it has to travel through to reach the player, all have an effect. I could describe it using a litany of engineering terms, but it's easier to (heh) hear it for yourself, in the video above!
Interested in trying it out for yourself? Click here for the mod's thread, including download and installation instructions. Enjoy!