Best answer by Stuart_W
1) Ensure that the Playbar doesn't receive audio signals whilst you are playing music via it's optical lead input - Maybe switch the TV off rather than standby? Or check if there are any audio settings. It's possible the TV is defective and erroneously sending out signals even though TV is off (Though you seem to suggest that you actually hear TV sound - Are you perhaps getting the audio connected to a PVR or other box and not the TV directly? - In that case switch that device off/investigate the audio settings as it may be "waking up" for recording/updates to schedules etc)/.
2) Change the settings in the room for the Playbar (advanced - Room settings) and set it so the Playbar does NOT auto start