I know there have been a ton of threads about this. I have an '09, and followed the instructions on this youtube video. It worked! I can put it in reverse in silence now!
I liked that gen 2, you could do it yourself. With modern Prius, you need an OBDII programmer, either yours or the dealers.
I'm probably the minority of people who like hearing the beeps. They seem to make me focus a lot harder on backing the car up, and it's easier to tell if I've made the full shift from reverse to drive.