It seems as though some enterprising individuals have managed to rip the music tracks from Rock Band. This might seem unremarkable at first until you remember that most of the in-game songs were based on the masters and were stored as multitracked audio, with isolated guitars, drums, vocals, etc. The files, which are saved as multitrack ogg files, can be easily opened in Audacity and easily manipulated. This is prime mash-up material.
This, on its own, isn’t that big of a deal. Multitrack audio like this is heavy and bloated and not really of interest to the average listener. These are of interest to obsessive completists, fellow musicians, and/or mash-up artists. For them, there already exists a shady underground network trading original master recordings (this is how those SongSmith versions of popular songs were created) so, for them, the Rock Band rips are actually low quality. The only thing of interest is new multi-tracks for songs that might not have had masters leaked, but I don’t know if this is the case here. That’s a community I don’t know much about.
But combine “rip multi-track audio from Rock Band” with The Beatles Rock Band coming this September and, well, things seem a little more notable.