I brought this exact same issue up with my SA at the last service. My audio even went into a mute condition once, only able to faintly hear music when opening up the volume completely. I had to take the key out and re-start the car to get normal sound output. Apparently it's a known issue. They fixed it with a S/W flash. It hasn't happened since.
2016 Agate Grey GT4 PCCB LWB PCM
2013 Mineral White e92 M3 DCT ZCP ZPP