To be honest, if you drop the "for use in DOSBox" part of your question, you'll get much better answers in "Marvin". We'll even move the thread there if you like. It doesn't terribly make your question different.
But:
- There are a few modern motherboards with ISA slots, mostly Intel based. These are usually called "industrial motherboards". They do not work with most classic soundcards that need DMA support (IIRC)
- There's a goofy ISA-to-USB adapter or two out there, I don't know if anyone's ever had the courage and money to test one. It's widely assumed it wouldn't work because drivers that allow direct access to the card (which your DOS apps would use, not Windows) would still need to be written.
- Either way you'd still likely need card drivers for your modern OS, good luck with that.
- Finally, DOSBox is a portable app and the solution you'd come up with would be OS-specific. You'd need a lot of developer help to make it cross-platform.
- If you need ISA support, you're better off building a retro rig. Fastest you can go has been debated around here before (I should find all the threads) but it's around Socket 478.
I'd just use DOSBox's Gravis Ultrasound emulation, it's pretty good, and if you already have a modern PC, you have the horsepower 😀
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Stiletto