This might sound really silly, but try using a simple name, like "egg.mp3"... although I expect the same results, to be honest. Otherwise, I'm kinda stumped, I mean, if the target actually points to where the file is, it should work. Have you known of someone that has tried to do something similar?
Or in the other hand, rename your mp3 file with the original name of the sound that you want to replace, and then simply replace your file with the vanilla one. That should work for sure.