Warren, there are a couple of options for playing CD audio, and you didn't mention which you were using.
Firstly, there is a DAC (Digital To Analog Converter) chip in the DVD-ROM drive that outputs stereo analog sound from the drive - but only if you have the 4-wire analog audio cable connected from the drive to the sound card. This is the default way to play CDs - and having the cable not connected or connected incorrectly is common.
If you are attempting to use the other option, SPDIF output to either the sound card or SPDIF passthrough from the sound card to an external digital receiver, you need to have the two-wire digital sound cable installed - some DVD-ROMs don't even have such an output.
DVD movie sound would not be affected by having either or both the above cables missing, since the DVD audio files are read from the drive across the IDE or SCSI ribbon cable.
More information, please - then we can help.
Gary