I have a proposed method of using the other voiceover sounds, and replacing the update one.
First off, all of the voiceover sounds are located in the '1033' folder in sounds. If they are modified, then (I'm pretty sure) they wont be used, so what you have to do is copy them around a bit.
Use my tutorial to copy the existing voiceover sounds out of that folder into the sounds folder, and then replace the update one with what you want. (Bob's/Sacha's if you want)
Make sure the option about voiceovers is off in the settings and it will use the ones in the sounds folder, even the voiceover ones.
(I also have the avast! 4 sounds if wanted...)