Apologies if this has been covered here, but after a quick look I can't see anything relevant.
Over on the Fenix 3 threads, there has been discussion about how the music control widget does not work with some (?most) of the listed music players under "Settings / Default Music App". This issue appears related to the GC app rather than the actual Garmin device.
Personally, I have only tested with Winamp... volume up/down works well, play/pause works sometimes and forward/back do not work at all as intended. Others seem to have found the same behaviour with other non-default music players despite changing the setting in the GC app to match the player they are using.
Is this a known issue? Can we expect a fix?