The behavior I'm imagining is: someone sends me a link to a song, provided by their music service of choice, which I can't use. I click the link, then my OS tries to open that resource in my preferred music player. I haven't thought about what happens if that doesn't work (not available on the service for example).

You're probably right that it would end up being more clunky and broken.

But I think a song is much closer to a resource (like a hyperlink to a website), than to a contact address (like a mailto link).

This is actually a regular problem for me, and I solved it by backing up my messages, then using a script to scan them for music URLs, and create a playlist on my music service of choice. It felt like a pretty silly thing to create.

If i open a .mp3 file on my computer, it will open my local preferred audio player. It could have the same functionality as treating the resource as a 'file'

There are already industry standard codes for songs, albums, performers, etc: ISWC, ISRC, IPI, IPN, ISNI.

Spotify already use some of those, but I dunno if they expose via API or make it searchable.