It would nice if it could do the above - you just want to play the sound track of a music video you have.
But it’s still a video file right?
I think a video player app like VLC is going to be your best friend here as you can set it to play content (even videos) on background.
Just lock your phone and listen 
Could do that, though it would be a nice all-in-one(app) feature.
Easier said than done no doubt…
Sorry no plans for that, metadata for those are rare on most media providers, and require transcoding and too many reasons it would not work perfectly and triggers more support than happy users.