Motherboard: MSI PM8M-V H/MS-7104 ver 3.0 (chipset: VIA P4M800 + VT8237R Plus)
Sound card: Turtle Beach Montego II with Dell part number, exactly as pictured in https://flaterco.com/kb/audio/PCI/#M2A3D
Drivers: I tried both w9x2041eng_drv.zip and Win9x2048.zip (the latter first); they both include the same AU30DOS.COM anyway

I did some testing in DOS prompts inside Windows 98 SE, in 98 SE's DOS mode, and with MS-DOS 5 (but not so much of the latter).
In a Windows 98 SE DOS prompt, DOOM 1.9 sounds fine, but in DOS I get music starting to play and then it gets distorted, and then I get a lot of screeching and crackling noises, making it unplayable.
I tried some other games and they work fine in both DOS prompts within 98 SE and in plain DOS mode: Wolfenstein 3-D, Prince of Persia, Microprose Grand Prix 2.

So for some reason it's only DOOM in DOS that is the problem.

By moving the card around between PCI slots, I can get it assigned to IRQ 5, 10 or 11, so I put it in the IRQ 5 slot. Nothing else is assigned IRQ 5, but the Sound Blaster emulation uses IRQ 7 instead and Windows won't let me change it to 5 (I could set it to 10 or 11 but that doesn't seem useful). Nothing else is using IRQ 5 or 7.

Does anyone have any suggestions about what I might be missing here?

Thanks in advance!