I didn't use anything other than that package I posted.

Did you copy all the files to a floppy disk in drive a: run the install.bat from drive a: ?

I ran it from a DOS command prompt (didn't boot into Win98SE)

It should have copied all the files to:
C:\OPTI931

It should have put these at the beginning of your autoexec.bat file
SET BLASTER=A220 I5 D1 T4
SET SOUND16=C:\OPTi931

And these at the end of your autoexec.bat:
PATH C:\OPTi931;%path%
C:\OPTi931\SNDINIT /b

It would have also put a line at the beginning of your config.sys file for a CD-ROM driver which you don't need.

Oh wow, reading was not a skill I practiced yesterday, apparently. First, I missed the bit about having to run install.bat! And second, I set the jumpers on the card backwards: I assumed pins are numbered left to right and top to bottom, but it was the exact opposite. Once I ran your script AND enabled PnP with the jumper, it works!!!!

Now I can enjoy Monkey Island's iconic music coming out of period-correct Braun L880 speakers through a more or less period-correct Telefunken HA880 amp 😀
BattleTech: The Crescent Hawk's Revenge also works!

Tomorrow I'll uncork a bottle of Australian wine and toast for you!

Sweet! Glad you got it working.