I believe there's only a > 1MB limit, but I don't do crap with music because it's just an excessively large download for likely annoying buzzes and crap.
I've never been to a server that uses Music properly or well.
People using custom sounds work, though. Buzzez and similar sounds if put in right, work really well, but, Music never really comes of successfully, especially not Music Songs. I just think they can't fit in with things right.
The only time I've ever heard music work right in Blockland, was a roleplay based around a NightClub, so, the music worked fine.
But, they were only short, although well-looped sections of songs.