Yeah, and I remember if you didn't have the audio cable that went from the CDROM into your sound card, you would never hear the tracks. It wasn't your computer decoding the music, the CDROM drive was simply doing the decoding, and passing the audio out into a CDROM input (just a line-level audio signal) on your sound card.

I'd forgotten about those little cables. If I recall correctly, they were small round cables, with maybe four conductors and a little flat connector with a clip?

4-conductor audio, generally.

Later on there was a generation of two-conductor cables that used SPDIF/PCM Audio instead of analog audio for better fidelity.

I vaguely remember CD drives with a headphone jack output!

(E.g. this one: https://pavel.network/using-cd-drive-as-poor-man-audio-playe...)

Audio tracks and WAV/AU files weren't actually decoded, but straightly send to the sound card as bits. That's it, a command like

        cat file.wav > /dev/dsp 

Would just work under Unix (/dev/audio under plan9/9front)

Thus, making a function for Quake to read the CD bits to the sound card would create almost no CPU usage, even less with DMA.

My cdrom drive also had an audio jack on it, so you could plug headphones directly into that if you wanted to have just CD audio and not the sounds coming from the PC.