Currently if someone plays something in BrainzPlayer (for instance, on SoundCloud), we run a search for the artist and track names in SoundCloud.

Before running this search we should check if that MusicBrainz recording has a URL 'stream for free' relationship to SoundCloud. If yes, play the linked SoundCloud song. If no, run the search as currently.

If the user has multiple services enabled, it should check for URL relationships for all of them, in order of "Music Services priority".

Clarification: It should iterate through every enabled service, checking for a URL relationship, before it resorts to running a 'artist + title' search on the first service. That the user has enabled multiple services implies (I feel) that they want the best result across all of them, before falling back on a basic search for the first service.