I don't have them any more, they were deleted along with the rest of my Avast 4 installation.
You would have to re-download Avast 4, uninstall 5, install 4, copy the sound files to another location, uninstall 4, install 5 again, rename the Avast 4 sound files to the same names as used in 5, then add those files to the folder I mentioned above, answering "yes" each time you were prompted "confirm you want to replace (this file) with the new (old) version.
Lot of faffing around, I'm fairly confident it would work.