Back in the DOS days I used to very much rely on the old ISA SoundBlaster (i.e. SB 1.0, 2.0. etc.), and it used to be very important not to buy some random sound card that only had Windows drivers but didn't work in most DOS games, but instead buy something actually hardware-compatible with an original SB. It goes without saying that the SoundBlaster also worked in Windows, which then was DOS-based, so good SB-compatibility gave you the best support in Windows 3.1 and DOS.
I don't actually know if there ever was a PCI card whose hardware was sufficiently ISA SB-compatible for it to work without additional software kludges/crutches, or whether that's even technically possible, but I do read that some later SoundBlaster PCI cards offered
a TSR that made them sing in DOS (and thus Win 3.1). So maybe something like that could work for you.
Of course, there always was the odd DOS game that only supported sound hardware xyz but not one's own sound card, but that being said, AdLib and SoundBlaster used to give you the widest support – remember, SB hardware compatibility implies AdLib support, because the SB was AdLib hardware-compatible. Windows 3.1 supported AdLib and SB, and games that didn't support either but supported something else other than the PC speaker were in the minority and often little more than vendor demos from competitors.
...
I just did some further research, because I wondered if there ever was some kind of PCI-to-ISA breakout box, or whether such a thing would have been technically possible.
Turns out there were such solutions... kinda... with limitations... lots of caveats... and very expensive:
http://www.accesio.com/?p=/pci/pci-isa.html (also google PCI-to-ISA)
But during that search I also found this:¹
http://www.karbosguide.com/books/pcarchitecture/chapter25.htm#RAID1
This would seem to imply that there were/are actual PCI sound cards that were/are hardware-compatible with ISA SB. However, I don't know which they are, and I don't know if Creative Labs themselves ever made them. (I.e. I don't know if any PCI SoundBlaster brand cards are genuinely ISA SB hardware-compatible. It
says here that "SoundBlaster Live" and "SoundBlaster PCI" were
not ISA SB hardware-compatible – they relied on the aforementioned TSR, which that post warns didn't work with all DOS extenders.)
Of course, instead of spending a lot of time and money on finding and buying a "Goldilocks" sound card², it might almost or even be easier/cheaper to just purchase a mainboard that still includes ISA slots.
¹
No, Fig. 170 isn't a PCI-to-ISA riser, it's a RAID adapter. It had me fooled at first as well.
²
Could this be any guide? Caveat emptor.
