Which music app uses MPRIS?
The only one that partially worked for me was QuasarMX.
I could see what song was playing, but I couldn't control it.

Also, seems like you've forget something that at the music control watchapp. When you hold down the Play/Pause button, the Next/Previous track buttons will change to Volumpe Up/Down control.
Will you also add this to your support?
By the way, My Pebble fw is on 2.3

(Note: I've already re-added my Pebble within the QuasarMX app, but still doesn't work )