As quoted by GameboyRMH,

All you need to do is...

While playing a track, delete the widget (this will bring the music to a stop). Then go back to your media player app and resume the track. Then re-add the widget and it will work.

There is a 50/50 chance that it will remove all your widgets (due to some hildon bug) but you can just re-add these as normal.

Nath