Oddly specific question but I just got an iPhone 6S and with the memory upgrade I decided to download all my music from the google music app into the phone so I don't have to stream and save a little data. However if the phone is plugged into the car via usb I get the "unplayable file" error message in my car. Playing through bluetooth works just fine though.
I removed the apple music app and my car will then just say "no audio file" even though the phone will show that it's playing music through the google music app.
I had a similar issue with my old iPhone 6 where even if I was streaming it would take a few tries to get the music to play because it would say "unplayable file" but after that it would work but if I tried manually changing a song in a playlist it would go back to "unplayable file" After months this finally stopped being an issue.