Well, it might just be the difference of FM music driver, since the hardware is different.

SB is single OPL2 chip, while SB PRO is either two OPL2 chips or single OPL3 chip (for SB PRO 2).

So you're suggesting that maybe the UU1 driver is intended for SB Pro 1 (two OPL2 chips) and maybe doesn't sound as intended on an OPL3 chip?

UW1's SOUND\SBPFM.ADV indeed is designed solely for 2xOPL2 SBPros. With an OPL3 chip, you'll only hear centered what is supposed to be the right channel. You could try taking SBP2FM.ADV from another game and renaming it to SOUND\SBPFM.ADV, and then also copy SOUND\UW.AD to SOUND\UW.OPL. This makes the music sound right on OPL3, but makes you lose the sound effects for unknown reasons.

Yes it definitely tries to write data to both chips in an interleaved fashion, so first the index write to chips A and B, and then data to chips A and B.

One subroutine writes identical index/data to both chips and another subroutine can write different data if it wants to.

Yes, that can't work on an OPL3 this way. It needs some modding 😀

Thank you, guys. That makes sense, and it sound perfectly find in regular SB mode. Besides, I'm almost done with UW1. On level 8 and just killed Tyball and saved the princess. 😀

Interesting. I had the older SBPro back then and I remember noticing that the Underworld music sounded a bit messed up after I upgraded to an AWE32. Ever since then I don't think I ever heard it sound the way I thought it should, including in youtube videos and emulators. I guess this explains what happened.