If the GMs catch you doing this you get banned.
You can get banned for replacing music files?
I doubt it. This is more of a client thing, the files for the music are located in
C:\Program Files (x86)\Steam\steamapps\common\Spiral Knights\rsrc\sound\music
So yeah, this doesn't modify anything in any way really, all you do is toss your song in (I think you have to make it an ogg I'm not entirely sure) and rename it to one of the other files, so like if I wanted to listen to the Chao Lobby theme from Sonic Adventure 2 or the Final Fantasy 7 Fanfare, I can toss it in there and rename it "haven" or "fanfare_elevator_1"
Thanks Flat for making me actually check into this because it's a neat idea. I'd make a backup file with all the actual songs though so then you could just toss em back in when you wanna hear the game's normal music.