I did the same thing once....totally freaked out but figured it out.
What you need to do is use SSH & Cyberduck (or rather) and go into Home/System/Library/Audio/UI Sounds and replace the "unlock.caf" file with the stock unlock.caf file.
Once you do that, it should be ok. I don't like changing the "sounds" because this happens.
Hope this helps.
Tried to attach the file but can't.....can't attach "caf" files.
Check downloads section for stock (default system sounds).....or private message me thru MMi.