I'm trying to do one of the bonus levels in DK1, and the redundant voice messages are driving me crazy. However, I can't turn down the sounds volume because I need to be able to listen for boulders rolling.

Is there a way to turn off the voice messages without interfering with the other sounds? Thanks in advance.

I hacked together an empty speech file. I did a quick test on Eversmile and it seemed to work.
Put this in Dungeon Keeper's sound directory. Make backups!
DkNoSpeech.zip

You can also use the DKSOUND.EXE utility from Dungeon-Keeper.net. It let you disable specifically those sounds you do not want to hear.
Dungeon-Keeper.net has been down for a while though, so download it from KeeperKlan.