This happened to me on the first day of ownership. (2024 Macan 4 EV) The car became bolluxed up when attempting to add my wife as secondary user (stuck on loading accounts). I got on the phone with 1-800… and they approved the secondary account, having me then delete and re-add my primary...
I also suspect the .m4a files that do not play are DRM protected. The .m4a file I tested with (see post above) is not. That file does play on other devices (Mac QuickTime Pro, Sonos).