On starting the game I can hear the sound but the screen is black.

I have tried editing the CFG file but it will not let me save/edit the file.

Is this not compatible with Windows 8?

I have also tried the compatibility settings but no luck.

I haven't installed it on Windows 8 yet, but previously I had it on Windows 7. The intro movie was always black, but once that finished the game played fine. If you skip the movie do you have any luck?

Right-click on the file and go to properties. See if the "read only" option is checked. If it is, unchceck it, then try editing the file again. It should work.

Alternatively, you can try logging in as admin if you're not already, and editing the file then.

I didn't buy this from GOG (I bought part 1 from GOG, and GOG didn't have part 2 available yet when I decided I wanted to play it), so no guarantees on whether this might work...

The game seems to work in Windows 8 in windowed mode, but not full-screen mode. The difficulty is that the game is set to full-screen mode by default. So use Waltorious's instructions to be able to write to cfg.txt, and change window=no -> window=true, then start the game.

Note that GOG buyers had a long history of not being able to see the opening video. So if you follow these instructions and still see a dark screen, you may want to wait a bit to see if the opening menu pops up.