This is a really great idea, kudos!
You might want to look at Docsend for feature and marketing inspiration - they started by allowing mainly startup pitch/sales decks to be tracked in much the same way your audio clips are, but have since expanded to a wide variety of enterprise use cases. I can only imagine that the music industry has many such use cases, including enterprise-style auditing/reporting needs and the ability to limit an audio file's distribution to within an organization. Not for the faint of heart to approach that, but there could be real opportunities there.
Some other random thoughts: This would be a perfect UI to standardize the pattern a lot of YouTube ads do: a ~5 second teaser clip that pre-plays the full material. With this and other network features as you grow (say, "submit demos to my SongBox drop box, and then I can quick-review my inbound SongBox feed"), you could gain the reputation of - "if I receive a demo on SongBox, it means that the sender is being highly respectful of my time." You might not want to build all that social-network functionality, of course, but it's a possible direction to go!
Also, custom subdomains (and beyond just removing branding, letting people add branding) could be a high-ROI way to add another pricing tier!