Update on the PCI Express CMI8738-LX situation.
It turns out PeekPoke was actually writing to the proper registers inside the CMI8738-LX and what I was seeing with RWEverything was actually, according to the CMI8738 datasheet, the PCI configuration spaces. Still, even with OPL3 enabled, port 388 is still closed with the PCI Express CMI8738-LX.
Then I decided to try using one of my PCI CMI8738-LX cards to force port 388 open. With both CMI8738-LXes running at once, both with OPL3 enabled via PeekPoke (verified with RWEverything as well) and with port 388 open, the only card that would put out FM sound was the PCI CMI8738-LX. The PCI Express one was completely silent. This makes me think there are one of three issues at play here preventing OPL3 from working:
1. The PCI Express buses in the test computers are not allowing anything through port 388 (I tried on a Lenovo ThinkCentre with a Core 2 Duo E8400 and an HP Workstation Z420 with a Sandy Bridge-E Xeon E5-1603)
2. The ASM1083 is not letting anything through port 388
3. The FM block is faulty on the PCI Express CMI8378-LX
I'm not sure which of these is the most probable issue, but considering user Soeuni managed to get both a YMF724 and CMI8738 working through an ASM1083 bridge on one motherboard while being unsuccessful with this same PCI bridge on another motherboard, I'm thinking the PCI Express bus has no access to port 388.
I only have three other computers I could test this out on. The first is an older Lenovo ThinkCentre running on a Core 2 Duo E6420 (upgraded from a Pentium D), the second an Acer Aspire with an AMD Athlon X2 (not sure what chipset this runs on, but it does have ATi on-board graphics, if that helps - I will check the exact chipset with CPU-Z) and the third is a custom-built PC from 2007 running on a Core 2 Quad Q6600 (this computer had the aforementioned Core 2 Duo E6420 prior to being upgraded to a Core 2 Quad) with an ASUS P5B-VM motherboard (Intel G965 Express chipset). If it doesn't work on any of these, I think the FM block is broken on the PCI Express CMI8738-LX.
Creator of The Many Sounds of:, a collection of various DOS games played using different sound cards.