I have this gaem on steam and from what i understand its the same version so I just want to share what iv done to my copy to make it run perfectly.
1. run it full screen at the highest res i can and turn of`f HW Vertex Processing andW buffer.
2. run it in xp compatablity mode.
3.d/l the patch
4.turn off antialiasing
and as a result iv never had an issuse with audio video or slowdown but it took some experimenting to figure this out. thought it would be nice to share.

The GOG.com version includes a run.exe hack which solves the audio syncing problems. The only problem i've ever had were some weird lightning bug which was solved by turning down AA to 4.