Go to your System Folder -> Preferences folder. Trash the "Apple DVD Player Prefs" file. Then go to the Apple Menu -> Control Panels -> Extensions Manager. At the top there is a popup menu labeled "Selected Set". Pop it up and choose "Mac OS 9.0.4 All". Then reboot.
Trashing the preference files of suspect applications is a good way to fix many problems. Also, using the Extensions Manager to turn off 3rd party extensions (drivers etc) is another way to track down conflicts. By selecting "Mac OS 9.0.4 All" you have just turned off all 3rd party extensions, leaving just the Apple-supplied stuff.