Excellent website for this topic: https://www.parkytowers.me.uk/thin/hp/t5520/

Sound is AC97, this isn't intrinsecally a no-no when it comes to DOS compatibility but it's often less than ideal;
indeed looking at that page and Using VIA VT8231 southbridge for sound blaster pro emulation (edit : sound blaster emulation available using software !) the VT8233 is a no go... (meaning PC Speaker only)

"The Pi thing" is probably using it as a MIDI synth, you'd need a MIDI output to use it which you probably won't find on any Tiny/USFF class computer (except some of the few that can take expansion cards if you provide one), even on a "fully loaded" full size microATX system just before the turn of the millennium the joystick/MIDI port wasn't universal

Yes, some programs won't like a processor that fast, there are some relatively effective slowdown programs and you may have underclock options in the firmware (almost certainly not in this case) for that but ultimately (if we excuse stuff programmed when the IBM 5150 was yet another computer invented for the sake of being different with probably no future, which BTW are out of reach of said slowdown methods) it's the fault of whoever made that program 😀

Conclusion - You probably want something else, on average older, or even look at a different class of product like industrial motherboards (whether backplane type or PC104) where good compatibility with vintage software is often a design point (as opposed to a side effect of using a low cost chipset with old integrated graphics and IDE only in 2009, as is the case with my Igel M310C!)