Update:
SoundBlaster AWE64 Value with Unisound works great for the problematic games. The ISA Configuration Utility also found the card and gave it IRQ5 and didn't allow me to choose IRQ7. Interesting...
I prefer to go to the SBPro2 honestly if possible, as I'm one of those who enjoy original OPL3 sound. I'll try some more, but it seems this mobo has an issue with non-PnP sound cards.

SB16-PnP also works fine. Tried hard tinkering with non-PnP cards but they all fail the same. So this is it.. off to find a PnP card with original OPL3 on it.
Hopefully I land on a cheap Vibra or SB16 PnP version that still has an OPL3. I don't care about MIDI as my original intention is to use a dedicated MPU card for MT-32 anyways.

I have the older model without soft power before they called it Optiplex on the front case. It also might have a different chipset so not sure how much help this is to you but they probably have a very similar Phoenix BIOS for whatever that's worth. The cards I've used all have true OPL3. None are sought after nor expensive. I've experienced similar but not the same stuff as you. Mainly Duke Nukem 2 would outright freeze the computer with music and sound both enabled. This was an ADPCM bug in the sound blaster's DSP. The non PnP ones for me were the only ones that suffered from this as they had a particular DSP version I can't remember off hand. I had a couple of games where sound straight up wouldn't work - Wolfenstein 3D - and only on the SB16 value. Replacing/flashing a patched DSP seems to have fixed everything I was experiencing. Also, a lot of games have patches so you don't need to do any hardware mods but they're buried in posts here over the years so it can take time to find them.

The cards I have and have tested are:
SB16 CT2230 - Not fully PnP, a bit noisy until I swapped the DAC.

SB16 Value CT2770 - Decent card but I've had one short out and die (the 9V regulator for the amp).

Vibra 16S CT2860 - It's a later PnP card but it's very quiet for a sound blaster 16 and nothing ever crashed with this one for me. No option to get rid of the MIDI bugs nor does it have a Wavetable header but luckily for you neither of these are requirements. Also, I definitely remember being able to successfully change the IRQ using SET BLASTER when figuring out IRQ conflict woes with SCSI.

Lemmings 2 gave me a lot of grief but not with sound problems. It was actually the integrated Tseng video caused all sorts of graphical corruption and random crashes. Can't remember exactly what the solution was.

Another benefit to replacing the DSP chip with a patched one is bug free MIDI but sounds like that's not something you need.

Thanks for confirming. I ordered a Vibra S CT2890 with OPL3 onboard to evade the problems. These later SB16s have compatibility issues with Sierra games and need an updated AUDBLAST driver to fix them. But as you said, it will be down to patching the games.. which is not terrible, just that I never had to do it in my previous custom 486 DX2 build.. that one spoiled me into thinking that all 486s are fully compatible with all software from its time. But I wanted to try a branded build because.. you know.. it has "style" 😅

I'll report back here once I get the Vibra S installed. And yes, I won't be running midi through it. That will go via MPU-IPC-T to an MT-32. I tried that already and thankfully it works fine.

Got a VibraS installed. It's PnP with OPL3. Alas, it behaves like all the other cards. On MK and Lemmings 2, no FM/no SFX even if the games claim they initialized sound successfully.

The only thing that seems to improve the chances of making those two games work, is to switch the CPU Speed setting in the BIOS from 66 to "Compatible". You can also toggle this using Ctrl+Alt+\ keyboard shortcut, and the machine will beep either a high note or a low note to indicate the current speed.

Anyways, with speed set to compatible, about 90% of the game runs result in good audio. Of course the PC becomes too slow as well, so I want to toggle back the speed after the sound init routines are done. The challenge there is to be able to do the speed toggle. Since there is no hardware turbo button, I am left with the key combo. In a game, this combo rarely works (I guess many games don't chain back the keyboard interrupt).

I discovered certain times in Lemmings 2 and Mortal Kombat where the speed switch is picked up. So I restore the speed at that point, and sound+music keeps running fine without issues.

I don't know why Dell calls this "Compatible" mode. Why would they make an incompatible mode in the first place? The mobo also only accepts a 50MHz or 66MHz DX-2. It refuses to go after POST if you put a standard 33 DX CPU (says a weird error on boot: Shutdown Failure). So I couldn't try considerably slower CPUs. But 50MHz also didn't solve the audio issues.

Any idea what compatibility mode is doing here? Those games run well with audio on much faster machines on all SB cards I have, so it's not something wrong in the games.

For the kicks, I probed the ISA bus clock and it was a solid 8MHz in both 66 and Compatible speed settings, just to rule out the possibility Dell is overclocking the ISA bus in 66 mode and causing compatibility issues due to that.