Wow, Duke 3D!! The BEST 3D shooter of ALL TIME!!! I'm serious. I loved that game to death. Played online deathmatches until my hand deformed into a mouse shape, lol. Eventually, I upgraded to a fast Pentium (for the time), and it wouldn't run anymore. I spent months trying to get it to run but finally had to give up. Anyway...
Sounds like you are almost there. Grats on getting the card to work (almost). It sounds like it is playing .wav files fine (those are the spoken voices and explosions (usually)). The prob seems to be midi. Seems like the midi info being sent to the card isn't being interpreted correctly by the card. If the music seems to cut in and out, it might be an IRQ conflict. But this wouldn't cause the distortion you describe (I think). So, you could try a different IRQ. Doesn't Duke have a built in option to set the sound card parameters? (A=220, I=7, DMA=5, for example)? Experiment.
I'm not sure about the distortion with the high notes, etc. I'm thinking it is probably the driver. In the Autoexec.bat file, what driver is being loaded?
Will look something like: set Blaster = c:\sb16.exe
Where the sb16.exe is the driver file.
Do you know the manufacturer of the sound card? Have you checked their site for DOS drivers? If that fails, try out a couple Creative drivers, such as SounBlaster, or Soundblaster16. They are still available for download at the Creative website:
http://us.creative.com/support/downloads/
Please let me know if you get Duke working. I might re-try it myself when I have some time.
Donk