Did you try lowering or disabling hardware sound acceleration in dxdiag? (click Start | Run, type in dxdiag and hit Enter, then switch to the sound tab)
If you reboot your computer, does that help? Someone else encounter this problem but rebooting cleared it up.
Try deleting all of the files in the '
..\Divine Divinity\Run\Dynamic' folder.
This folder is used as a cache for the game. Files are created / copied there from the save game folder when you load, and written / copied from that folder when you save a game. The files will be re-created as required the next time you start Divinity. If there was a corrupt file in that folder, deleting the files should fix the problem.
If you play with the volume for the different sound categories, does that effect the screeching? The sound during the blessing ceremony is from the video, though....
Have you tried backing up your saved games and doing a clean install? (see
Troubleshooting Tips)