That is really strange. I tried Descent's setup program in Win98SE, and it plays on my synth just fine through my sound card's gameport. I'm trying to think of why it doesn't on yours. Did you double check the address? It looks correct in that video though...
Edit: Oh, I just remembered right after I posted. The card I'm using is an Audician 32 Plus, and I have a separate configuration utility in the control panel for it that allows me to select MPU401 Out for DOS application, either external or internal. Depending on what card you're using, there might be a similar utility for yours. That is probably what you're looking for.
Pentium MMX 233 | 64MB | FIC PA-2013 | Matrox Mystique 220 | SB Pro 2 | Music Quest MPU Clone | Windows 95B
MT-32 | SC-55mkII, 88Pro, 8820 | SB16 CT2230
3DFX Voodoo 1&2 | S3 ViRGE GX2 | PowerVR PCX1&2 | Rendition Vérité V1000 | ATI 3D Rage Pro