Just yesterday I was investigating music/production startups, and one of the few ideas I came across that seemed halfway-interesting was Splice[0]. On the producer side they have plugins for a few DAWs that enable easier collaboration and version tracking and so forth. But what comes of that (and why it's relevant to your comment) is a Soundcloud-ish embeddable web player that breaks out all the tracks/stems into a timeline beneath the main player interface.

It's pretty interesting, although I don't know how much of an opportunity is there, for some of the same reasons mentioned in the other replies. Musicians and producers can be incredibly reticent about this stuff.

[0]http://splice.com