You have to have ALL of the stock game sound files in your mod's Sounds folder in order to use them. SE4 will NOT look in the stock Sounds folder for missing sounds. Alternatively, you can place the new sounds in the stock Sounds folder...
Edit: Posted this before I read Ekolis' post... silly me.
There was some discussion months ago about creating a Sound Pack, just like the Image Mod but for Sounds instead. Unfortunately, there did not seem to be much interest in making submissions to it... maybe it is time to start it up again?
[ July 30, 2004, 00:05: Message edited by: Imperator Fyron ]