AC'97 and HDAudio emulation

[Torinde]

VirtualBox 6.0.0 has "support for surround speaker setups (as used by Windows 10 Build 1809)" - I assume in HDA, but maybe also in AC'97? Hardware HDA was supported by WinXP/2K and AC'97 also by Win9x.

May help also for #3597

Originally posted by @Torinde in #552 (comment)

86box has AC'97
QEMU also emulates HDAudio.

HDAudio open source drivers for DOS (mpxplay, SBEMU), Win3x (along with AC97) and Win9x (instructions, discussion)

#4716 added support for OPL stereo-extension, 4-ch output.

DJGPP supports HDA for Vortex86EX:

there’s sound library for DOS/DJGPP that supports HDA and also there are few DOS game engines newly compiled with HDA support: Quake, Quake2 and ZDoom.

Related: schellingb/dosbox-pure#449