Yeah that's a good idea never heard of that before. It would be good music plays when you are close to a mineshaft or stronghold and think that would be great to add! Sometimes you here like creepy sounds but I think music would be cool to add like when you are close to a:
I think it would be cool if they would add more music disks to minecraft at some point.
Although I wish that if the main minecraft music was playing and you wanted to play a music disk as soon as you put it in the main minecraft music would stop so you could listen to the music disk instead of both.
On the adding your own music point, what if there were 'music packs' like texture packs and you can play different playlists depending on your taste of music! I don't know, I just thought that would be cool!
Sometimes I feel like listening to the Minecraft music just on its own when Im not even playing minecraft haha. The problem is, now because of obfuscation by the developers, it's way more difficult to find the music files. Oh well... I could probably just find them on youtube or something if I wanted to. It's always happy pretty music.