Sound setup

The audio map across all three OSes.

The core problem

The onboard VIA HD audio works in XP and Linux but is useless in DOS — no driver, no Sound Blaster/AdLib compatibility. The nForce chipset also doesn't properly support the legacy DMA that real-DOS Sound Blaster digital audio needs, so even a PCI card struggles with digitized sound. FM/AdLib music (no DMA) is the reliable part.

DOS sound options

PC speaker — already wired to the case. Mono beeps, works on power alone. Set games to "PC Speaker" output. Free, works.
Yamaha YMF724/744 PCI — FM/AdLib music works (port 388h). Sound Blaster digital audio is a maybe via the DSDMA/ETEKVM TSR. Decision pending.
DOSBox-X (on XP/Linux) — emulates SB16/AdLib perfectly and fixes timing. Best route for playing games.

CD audio-in (connected)

A 4-pin analog cable runs from the CD drive's rear ANALOG AUDIO header to the motherboard's CD-in header. Press Play on the drive's front panel and its analog audio reaches the onboard codec and the speakers.

If silent

Un-mute the "CD" channel in the mixer — alsamixer on Linux, the volume control on XP. It's muted by default and is the #1 cause of "no sound". If channels are swapped or dead, flip the connector 180° on one end.

If the Yamaha card is added

Install it in a PCI slot (not PCIe).
Plug the speakers into the card; use it for all three OSes.
Keep onboard audio enabled — otherwise the CD audio-in path dies. The Yamaha coexists fine.
Linux: snd-ymfpci loads automatically. XP: install the Yamaha 2000/XP driver.