I was so confused by the CD drives of that era. They all had a volume wheel and a headphone jack, but never once did I experience those working. The audio CDs were always “owned” by the OS, which piped the audio through the normal channels out my speakers or the PC headphone jack.

I imagine the existence of those means that CD drives had their own DAC and other logic. I guess there was an idea of wanting to play CD audio without it being a PC concern? Or on PCs without audio capability?