maybe the block has the beat and instrument/sound all ready depending on the type of block and you just put the melody and simple effects into it e.g. fade, volume as this would make it quite easy to use.
Perhaps some sort of in-game recorder, and if you couple that with instruments (playable ones, please ) then you'd be able to have your own little music broadcast network. Quite interesting, especially since a guitar was seen in a screenshot of a character.
It'll be nice if you could choose "instrument" and "note" for these blocks. Even better if you could manually choose the frequency and letting you change frequency with mechanisms, letting you use notes that are not standard and change them on the fly, so people can do stuff like dubstep.
I'm totally going to mod in a radio system, broadcasting an .fm based on radius. Not to say the in game music won't be spectacular already. I love the homestuck albums.