you have entered my domain muh-ha-ha *sinester mad scientist laugh*
are the sound in the correct wave format (did you convert them from *.aud to *.wav)?
did you enter them in the sound(md).ini?
where did you put the sound files?
(did you put them in the audio.bag, using the XCC Sound Editor that is built into XCC Mixer, or did you put them into the game using the XCC mod launcher?)
Those are the methods that I am aware of.
Glad to be of help!
Edit: Did you use the same upper and/or lower case with the names?
Remember the ini's are CASE Sensitive