Oh thanks this is real bad, seems like Microsoft representative when I called their service side, told me their tablet does not function the background music when tablet goes to standby or sleep mode. As their application going to suspend mode ( which means in hibernation ) hence the power is reduced down... and that affects the music playback on background mode.
I not sure if ms Representative is giving me a correct answer hence I asked you guys who have the set. Is this a glitch on hardware or software ? (

no point holding my set if it cannot do playback )
Well I hope they can iron out this issues.