I found the following hack on how to turn off the back up beep in the Toyota Prius. Source:Disable Backup Beep - GreenHybrid
1. Power on the car to IG-ON or READY. IG-ON will do for this purpose.
2. Using the Trip/ODO button, set the Trip/ODO display to ODO (not Trip A or Trip B)
3. Power off the car.
4. Now power the car to READY (brake on). This is required so that step 6 works correctly.
5. Within 6 seconds of powering on, press and hold the Trip/ODO button for 10 seconds or more (there is no indication... just count)
6. WHILE STILL HOLDING ODO *after* the 10 seconds, shift the "gear" selector to R, then press the P button. Now release the Trip/ODO button.
7. If the last step was successful, "b on" or "b off" should be displayed in the location where the Trip Odometer or Odometer is normally displayed (front dash area). "b on" is beep on (default), and "b off" is beep off.
8. Press Trip/ODO to toggle the mode.
9. Now power the car off to exit the toggle mode.
10. Power the car on to READY and confirm the reverse beep status by shifting to R. The beep should not be audible if "b off" was selected, and should be audible if "b on" was selected.