A free app for Android, by Der AppKoch.

This app is a simple background service for Android. It detects any playback on the global audio and then opens the BluetoothSco channel so the audio is played on your mono Bluetooth device (hands free profile). This is the only app I’ve seen that uses the Visualizer API to grab audio information from the global audio and uses it to determine if the audio is played. The service uses the AudioStreamManager to start and stop the service. The redirection only starts if a bluetooth device is connected to the hands free bluetooth profile. If the hands free connection isn’t available anymore the redirection stops.

The service is only startable if the bluetooth adapter is turned on. The idea is to use it for navigation apps.