The results are still the same, even if I start mplayer from the command line using:
mplayer -vo cgx -ao ahi.dev -framedrop -dvd 2 DVD_DEVICE=ide.device:2
It plays the music video DVDs, or at least the pre-menu splash screen, fine, while it aborts with errors of not being able to find various conf files or the DVD if I place one of the other DVD's into the drive. I was using the icon to test playback since it was giving better playback (both resolution and aspect ration settings) than typing in the command line each time, as well as being able to easily choose which section of the DVD to play. Based on it's ability to play two of the DVD's like that, it would suggest there was a difference between those two and the rest of the DVD's, not an overall problem of having the software settings incorrect.
Pegasos I G3/600
MorphOS 1.4.2
128MB RAM
Toshiba DVD-ROM
20GB WDC Hard Disk
Radeon 9000
SB Live! LS 5.1 soundcard