I dont have any problem with the sound running them under W98.
I dont know if this works for Zak or MM but in games like Monkey Island you can choose using the internal speaker by running the game with "v i" Ei: monkey2 v i.
I: Internal Speaker
V: Vga Graphics.
You can also check if the sound if set on, by pressing F3 F4, cant remember, it was some F.