I've found that .mp3 versions of default files are being included in the .res if the sentences files include them.

For example:
!SC_HELLO0 scientist/excellentteam

this default file exists, but instead of a .wav resguy would look for an .mp3 and include some custom, random file if there is any resulting in scientist/excellentteam.mp3 being something not intended at all to be there.
I believe sentences files should only be detected as .wav

EDIT1: by the way, just in case, my default files text list has it included "sound/scientist/excellentteam.wav" perfectly just like all others, so its not a thing about it being missing from there