many of the newer aftermarket car audio head units will play not
only mp3 files, but also wma(windows), aac (iTunes) and some others.
Most older stand-alone CD players will NOT play even mp3 files.
It is a near certainty that if the players you are complaining about were made 2003 or earlier that they won't play mp3's let alone the other formats, and it's common that players any older than 2003
won't play burned discs even in cdda format (standard CD).
another problem area is with RW discs...
My 2002 vintage upper end Sony car head unit won't play those.
My 2006 Vintage CD changer will.