I get black borders around the screen in the Mac version of the game. Is there a way to remove them?

Do you mean how the OS automatically corrects the scaling to 3:4 for games that output in 3:4 resolution? To get rid of it, lower your monitor resolution to a 3:4 resolution eg 1024x768 and you will get a stretched image.

I actually use an old 4:3 CRT monitor for this game, so there is no need for stretching. :) But even though the aspect ratio is correct, I still can see additional black borders. I've attached a screenshot (I marked one of the borders).

oh, that's different. I was imagining huge bars on the side shrinking the screen. That has everything to do with the program itself. Are you sure the original game wasn't like that? I can see that the GoG version is infact a standalone scummvm package. (as such doesn't contain the original dos executable). Perhaps there is a way you can configure the standalone scummvm or take the data files and use it with the non standalone version, which will give you a little more configure-ability

Personally I don't know why they went with scummvm, but then In a world where dos emulators are as efficient with as many options they have for hardware, I never really saw the use for scummvm to begin with.
If you want you can go digging around on the internet as the game was released for free, get the original dos executable, and run it in boxer/dosbox. Perhaps that will display as you expect it too. Also look into mt32 emulation as this game was designed with it in mind, and you missing out with soundblaster fm sounds. (boxer has it built in)

I just tried standalone Scummvm, it gives you a few options at least. Also It lets you have soundblaster/voices and munt mt32 emu, which is nice. I can't stand the adlib track. The original dos version could do either/or which was a shame.

I haven't played the original version, so I don't know. However, on Windows Vista everything looks just fine, maybe it's an issue with ScummVM on Mac?

PS Thanks for the sound tips. :)

Well with actual ScummVM you can configure the resolution as a host of other graphical stuff.

I think I'll find some way to fix this issue. But still, it'd nice if the GOG team got rid of these borders in their package for the sake of other players.