Tapes.fm [beta] - A Music Collaboration Service

I haven't gone too far into it yet, but the workflow seems to be select a tape for remixing -> download track(s) -> add/modify track(s) -> upload and update the tape to match what you had in your own multitrack.

So you do get the bits, putting it together in the browser is essentially so others can listen to it and do remixes of their own.