I think Soundblaster only plays a precalculated buffer, mixing is done by software. Gus has onboard memory and uses mono samples which are mixed to stereo using panning commands. So now there are several possible reason for your problem. First, Doom might use stereo samples. These would take double space on a Gus as they have to be splitted into two samples. The Gus would also have to mix more samples and lose precision. Second, the sound library used by Doom is written without support stereo on Gus because of lazyness or whatever.
Midi is sepereated from Digital Audio on most cards, but Gus has no Midi. Gus uses Midi emulation by loading Midi patches into its ram and playing them the same way as all other samples. So the Midi emulation of Doom's sound library uses panning commands, the Digital Audio driver does not.
The final reasons could be:
-Lack of ram. DosBox only provides 256k on Gus.
-Lack of features. Dosbox might implement Gus without some features you have in later versions.

I believe that DoxBox v0.65 (and v0.63) have a GUS emulation that provides 1024KB of "RAM" for the patches, not just 256KB.

dvwjr

Thanks for the reply, but i don't think that's the problem. Games, generally, don't use stereo samples. They use mono samples and mixing (panning) to stereo by software (SB Family) or by hardware (GUS). As you said, maybe it's lack of current DOSBox features... But(!) why MIDI music is stereo? Since it uses the same audio channels for playing back music. That's what I don't understand.

Btw, I tried several other games: Horde, Full Throttle, The Dig, Rebel Assault 2 (they don't use MIDI music, only digital audio) and all the same: mono audio... 😠

I could use SB16 for audio, but GUS emulation is much faster, than SB16 and for few mid-90's games my rig is lack of horse power... (I have an Athlon XP 3000+)