I have recently downloaded the new Minecraft 1.5.2 pre-release and 13w17a snapshot. It has added a lot of new stuff that you probably already know about. But did you know Dr. Zhark is working with Mojang to add the horses. That is why they are so similar to the mo creatures mod horses. But What I wanted to say was, I was looking through the new files of %appdata% Minecarft and found a file called assets; which holds a bunch of files labeled icons, music, Lang, sound and records (The music disk). Now i haven't tried it yet, but i looked in the music file and saw a bunch of music with the icon you would use to play it on your computer example mine is VLC media player. So I'm assuming that you can take the music out and add your own to play in Minecraft as randomly as the music already does. You could probably do this with the other files like sound and records.
Just thought i would let you know in case any of you are interested in that kinda stuff.

yep this is possible, just make sure the songs you put in are the same length. I reccomend using a sound mod like atmos/auditori instead of doing it yourself