Which Media App plays from bluetooth?

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http://forums.androidcentral.com – I am a new convert to Android from iPhone and I've started off with a Nexus 4. I have what is probably an obvious question: When I use a bluetooth audio stereo headset to listen to music, if I press the play/pause button before I go into any app, music from my device starts playing (from the first alphabetically listed track). However it's not coming from the Play Music app because if I then open Play Music and choose a track and press play (it's not a pause button when I enter the app) it starts playing that music ASWELL as the audio already playing from the bluetooth press. My question is (HowTos)