The Tomtom app uses whatever is configured in Android's for media output. So the goal is to get the Android music player to play to your car over bluetooth (it requires A2DP like AndyP said). Once the music player works, the Tomtom app should too.
One thing to try: go to Android bluetooth settings and long-press the car's connection (or tap the gears to the right, depending on Android version). It should open up checkboxes, you need to enable media there.