Another thing you may try is to take a quick look at the audio cables connected to the burners as well as Window’s mixer settings. Media Player that comes with XP has the abilitity to play back audio CDs using only the standard IDE ribbon cable (no audio cable is needed) but if the drive does have an audio cable and cd audio is muted in the mixer settings, you won’t hear anything from it.
If neither drive can even read the audio CDs when attached together, but they can read data CDs more than likely, you have some sort of software issue. XP isn’t the most intelligent of OSes, it may be getting confused as to which drive is to be used for audio by defualt. I had a similar problem once. I have a Pioneer A05, Lite-On 48/12/48 (40x OCed), and a Samsung DVD-ROM. For a while, my system would only let me use the Lite-On for audio CDs (it would just ignore audio CDs in any of the other drives, with any program) I remember I had to unistall and reinstall Media Player, and that solved my problem.