Seems like a difference with SB16 FM (OPL3 music), not MIDI.

Yes, I think he means the adlib sound. I can confirm that apart from 0.70 this problem is apparent on both Dosbox 0.63 and Dosbox 0.65. Most tunes have an awful metallic distortion and this completely alters the music environment. One such very distorted track is the one played when you click on 'see dune map' and then click on the globe - the music played there is a good example of that awful metallic distortion that I am talking about. I agree that the soundtrack, if heard in original, is astounding. The problem lies somewhere in the adlib emulation. And Vdmsound isn't much better.

I suggest to try if there are differences between sbpro(and lower) and sbpro2/sb16 setting. First use opl2 emulation, latter are opl3. (can be also enforced by opl mode setting).

As some games rely on sensitive timing of OPL 2/3 commands and OPL emulation in dosbox has lower timing precision (vdmsound and dosbox use same emulation of those chips), sometimes sound is distorted. Maybe dune tries to play digitized sound with it?

I tried that and there's no difference at all. Nothing I do makes a difference in how the music plays. Adlib and Soundblaster sound identical though.

Maybe see if it sounds correctly in mess. If it doesn't it's most likely a
problem with the timing which is unlikely to be solved.

Also, MUNT's MIDI in Dune 1/CD ROM is really off. I suspect that it uses many custom SYSEXs to generate it's instrument map and I remember it sounded almost as wierd via MEGAEM on my GUSMAX. The SBPro dual OPL2/OPL3 stuff sounds pretty much spot on to me though - I reviewed the game when I was a journo back in '93 and played the absolute hell out of it too so unless my memory has faded badly I'd have to say that's it probably an issue with the games FM driver and the variance of certain SB16's. I seem to recall some had issues with their FM playback side - especially in terms of timing and pitch.