Jason, from an architectural perspective any XSPF player is a resolver. They have ultimate latitude to do what they think the user wants. But that’s not the answer to your question.
Who would be the resolver is any app that mediates between the user’s personal catalog and the cloud. Winamp, for example, both manages a local collection of MP3s and handles incoming URLs for multimedia.
You’d expect provisioning services to step up and write these. The makers of Rhapsody, the iTunes store, Yahoo Music Unlimited, or even LimeWire et al. Also, software that manages the user’s personal MP3s should make it possible to map song metadata to files on disk.