File formats for 2009 Audi A6 CD player

[Garry1]

I have a 09 reg Audi A6 which has a 6-CD changer in the glovebox (no SD card reader or Aux port). I have some audio files recorded in various seminars on my iPad which I want to listen to in the car but I don't know if they will play. Before I go to the trouble of burning CDs (and possibly wasting them!) can anyone tell me if these files will play in the car? If not, what format do I need to convert them too? They are all m4a files. Thanks.