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?
reply4-conductor audio, generally.
replyLater 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!
reply(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
reply cat file.wav > /dev/dsp
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.