1) Go to
http://u-boot.realsimulation.com/
2) Go to the "Audio" section.
3) Download any of the Das Boot Sound Mod files (of course I recommend Sailor Steve.rar)
4) The files go in the "Speech" folders in the "Sound" folder.
Alternately, if you saved your original "Data" folder before covering it up with mods you can just take the appropriate files from there and have the original game voices.
If you didn't do that, you should have. You never know what you're going to want back.