last time I asked the question some Blizzard rep replied that changing the Original files in ANY WAY was considered illegal. No wait - not illegal, just not accepted by the EULA. So If you want to do this, don't when playing on official Servers. Its too easy to do a checksum scan of those files.
i.e. I was specifically asking about using the english sound files (original) along with the german client (original). Nope. Wasn't even allowed to mix the original Blizzard files from different language clients. :/