Thats where you are mistaken. All soundcards can play cds via the pci bus - no need for any other connection between the cd drive and the sound card.
I've never been able to play a cd unless I have the drive physically attached to a soundcard.
When I play a cd in winamp 3 and I select the digital outs of my layla for playback, I get nothing. However, If I play an mp3 it works fine.
It is actually a really bad thing to connect a cdrom drive with a digital cable to a soundcard (for the few cards that have that option) because that means you lose use of your digital input for anything else and your clock is derived from the crappy clock of the cdrom.
I'm not really concerned with consumer cards, but for the sake of argument, my soundblaster live platinum has on the card 2 cdrom connectors and an extra digital connector (hooks up to the digital output of a dvdrom drive for example). I have all 3 hooked up on my work box. The breakout box has 2 spdif connectors as well (in and out). I can use all of these inputs at the same time with no problem.
Also, so far as I've experienced, when recording (or playing) through spdif, the clock defaults to the device doing the playing.
The current versions of windows media player and winamp3 have an option to to play cds digitally. Just select the option. I use winamp 2 and use the cd reader input plugin for the same purpose (do a google for cdreader).
i did dome digging on the winamp cd reader plugin. It seems to have bad jitter problems. Also, it's doing DAC on the fly - not exactly what I want from a reference cd player (I just want pure digital signal to feed to my layla and let it's DAC's do the work).
From the winamp website (this is what the plugin you mentioned bypasses):
Why doesn't the equalizer work in the CD audio and MIDI plug-ins?
Winamp plays these formats "passively", meaning the actual sound data never goes through Winamp. In the case of CD audio, it goes from your CD-ROM to your sound card through a cable inside your computer, and in the case of MIDI, your sound card actually synthesizes the sound from MIDI events recorded in the file. Either way, it's out of Winamp's reach, so to speak.
So... in conclusion, I'll report back on how the dj cd player works. I'm hoping the clock will be better than consumer gear, so my fingers are crossed...