I could see JSPF working well in music aggregators & semantic discovery applications.
Playlists also seem like a natural fit for the compact multimedia widget/card design pattern .
What's the use case for a track specific format? I feel one of the advantages of xspf/jspf is simplicity - it can handle multiples, but certainly doesn't require it. It would be interesting to support a "timeline" of annotations for a single track (sub-tracks, notes, links), similar to youtube annotations or soundmanager2's annotations example . Not sure if that is out of scope here though.