And you thought downloading .mp3 files, compressed digital audio tracks, was bad. "Bad" as in illegal, you shouldn't do it. Obviously I don't mean legal Mp3 downloads, just the illegal variety. Anyway, meet .MOGG, or Multi-channel OGG. Whereas MP3 is actually a closed format (despite it being widely traded), Ogg Vorbis is a free alternative. Legally, MP3 decoders (players) and encoders must pay royalties to use the format. Ogg Vorbis doesn't have this stipulation, so it gets used in games. Games that have music, games that have sounds - that covers all of them. Anyway, an OGG is just another kind of music file; see also: MP3, WMA, M4A, AAC, etc.
MOGG is different, though. Sidestep for a moment: When you miss a note in a Guitar Hero game, what happens? The game kind of belches at you, to let you know you messed up. That's because all Guitar Hero does is play the song, and it makes a sound effect (I think it's supposed to be a guitar string breaking?) if you miss a note. That's easy. But what happens if you miss a note in Rock Band? That *note* actually *doesn't play* but the rest of the song *does*. How exactly does that work? Well, Harmonix, the game studio that makes the game, actually licenses the master recordings of the songs, which are a bit more complicated than regular MP3 files. They have all the instruments separated, and that's how Rock Band works. It plays all the channels at once, and if you miss a note, it briefly mutes that channel.
Well, suffice it to say, some .MOGG files have "become available". If you're clever, you should know where to start looking. I cannot say more than that. Legal issues and all. I also know that efforts are under way to reverse engineer the game - I don't think it's so much that people want to get songs for free ($2 is more than fair for a song, considering how the game works) as that they want to import songs into the game that Harmonix hasn't brought out. (If I were involved in the project, some Nightwish would be going in there for sure.) Anyway, without the original master recordings, importing new songs won't be happening - unless people involved in the project are in bands themselves and can supply their own masters. A Rock Band downloadable song, it would seem, is one of these .MOGG files, plus the data files. 12 tracks - three instruments at four difficulty levels. Difficulty doesn't scale, so each difficulty does in fact get its own track. Plus the vocals and pitch/tone levels. Difficulty does scale for vocals (in terms of how far off the mark you can be) so that's its own thing. And the 30 second or so sample that plays when you're looking at the song on the list.
Anyway, I hear that these .MOGG files are quite fun to play around with. Not being much of an audio engineer, I wouldn't know what to do with them, but I imagine it should be trivial to mute channels. You could take out the vocals entirely and have an instrumental song. Be warned, they don't sound as good as you might think, vocals contribute a lot. Someone with Disturbed leaked pre-vocals MP3s of their first album, and when David Draiman is supposed to be singing, it sounds very... simple. Music with lyrics is, it's the solos that sound good because they're designed to be heard without the voice. The entire song is not like that when you remove the vocals. Anyway, you could also listen to just the vocals. Or the vocals plus the drums. Or, if you play an instrument (a real one, not a game controller that looks like one), you could just play that channel and jam along with it.
Your average audio-playing software (e.g. Winamp) won't play these files, but an audio-editor (e.g. Audacity) will.