Since there seems to be a number of posts discussing how to make Gabriel Knight 1 sound properly, I thought I could perhaps share a couple of conclusions I've reached about the subject, so people don't waste their time trying to fix things that can't be fixed. Of course, feel free to correct me if I'm wrong:

1) The voice recording quality in Gabriel Knight 1 is bad. The lines are full of cracks and pops, and there's nothing that can be done to fix it. I'm not talking about the clipping problems you can get if DosBox is not properly configured; I'm talking about cracks that ALWAYS appear in the very same places, in the very same lines. They are part of the actual recordings. They appear when you use DosBox, they appear when you use Endroz's XP installer; they appear when you are using the Sound Blaster sound driver and they appear when you are using the Thunderboard sound driver.

2) The game also appears to have problems when playing two different sounds at the same time (I'm not referring to background music). Basically, the second sound plays with some lag (some time after it's supposed to play). You can see this at the very beginning of the game, when Gabriel grabs the coffee machine and drinks some coffee while Grace is talking in the phone: the sound of the coffee being served plays like one second after Gabriel has used the machine. However, if you skip Grace's dialogue before Gabriel grabs the machine, the sound plays correctly. This problem is particularly bothersome in the climax of the game, because it's one of the most frenetic scenes and all dialogue lines play with some lag as well.

This lag problem could conceivably be fixed, although I haven't managed to find a way to do it. The voices problem, however, I'm pretty sure is not fixable.