I'm guessing by the mid-nineties, the cd-roms could send their sound data through the bus, but, I figured I'd ask for any thoughts.

Games that use Redbook CD audio tracks (where the tracks are also playable on an audio CD player) will require the cable to hear the CD audio tracks through your sound card .

Sending Redbook CD audio through the ATA bus was an option in Windows XP (and possibly 2000). It was not available in Windows 98/SE, as far as I know . It might have been available under Windows ME . It was never an option for DOS-based games.

Is this the WSS we're talking about ?
If so, I'm not surprised it lacks any CD audio inputs.

In that time period, proprietary CD-ROM interfaces were still common and
sound cards like the AdLib, PAS, SB 1.x or SB 2.0 often had no CD support at all. 😐

I recall my father's 386 PC had got a dedicated CD-ROM controller card with RCA connectors installed, too.
But they were just meant to provide a simple, passive pass-through to the internal CD audio.

Anyway, that card has a line-in jack. So it could be possible to make a little cable out of an
audio cable w/ jack plug and an old CD audio cable. Or so. Well, if the card has a mixer with sidetone feature..

Another idea would be to use the CD-ROM's headphones connector.
Either for connecting a mixer to it, the sound card's line-in (again) or some headphones.

Edit: Sorry for my poor English. Had to look up some words in my dictionary.. 😅
Edit: The WSS' RCA connectors might be intended as a line-out for a quality stereo system rather than for CD audio (just guessing).

For a soundblaster card, is the conversion of the rebook-audio-in to the output passive? What I think I can do is put in some cheap card with the connector, in addition to the Windows Sound System card. I'll then feed the sound into the audio -in of the MS card. No drivers for the sb card, it's sole purpose is to convert cd sound to the 3.5mm jack.

Yes, we are talking about the card linked to above.

The lack of redbook may be no problem. So far, they all sound to be cd versions of the game's general midi track. (Warcraft 2 is an example). The system will have midi interface, I can just use that. I want to hear these games on Yamaha XG anyway.

Hi, yes that could work. But beware, some of the later SB cards (SB 16) do use their mixer for routing CD-ROM audio.
Thus, they may require drivers and power to do so.

PS: There were also some cheap IDE controllers with audio-in and RCA or jack plug connectors..
They were also passive, I believe. So in theory, they could be mounted in a PC case without using an ISA slot at all.