If your sound card has two CD-ROM drive connectors (as does the SB64 PCI), and you connect cables from the card’s ports to each of your CD drives, you may find that you can only get CD audio from one of them, even though you can access data from both drives simultaneously. This is probably a limitation in the current sound driver for that card. Until this is worked out, make sure the CD-ROM drive you use most often for audio playback is connected to the outermost port on the sound card.
If you’ve only got one CD-ROM drive and you’re not getting any CD audio, try moving the cable to the card’s other port.