So I just fixed the sound issue on MY Spy Hunter last night. Look at the sound board (lower right side inside the back box). Mine says "Cheap Squeak at the top of it). I had no sound until I looked very closely. At the center or so of the board is a tiny little knob. It looks like a black gear and is only about the size of pencil. I played with that a bit and boom, I have sound now! It's worth a look.
Good luck it's a neat game!
Keith