Welcome to Android Central! Are you talking about a car stereo system?
First, make sure the volume on the phone is turned up. If the volume is turned down fairly low on the phone, it may seem like there's nothing playing on Bluetooth stereo system.
If the volume is turned up and you still don't hear anything, then go to the Settings>Bluetooth menu, tap the gear icon next to the car stereo listing, and make sure Media Audio is turned on.