Car bluetooth Problem
A common issue is the output selection in the media apps: 

Make sure that on your phone, in the media screen, you have selected the correct output source (Bluetooth - Mercedes name) to play the audio (assuming audio is coming from the phone when it doesn't work)

If no audio comes (make sure volume is up) .. However, the Mercedes model / year may not support music playback. 

As @tyson mentioned, inquire with the car manual for Bluetooth settings