I have a 2005 Audi saloon shortly after buying it the sensors went silent no beeping at all, I listened to the individual sensors when ignition was on and car was in reverse the drivers side corner wasn't ticking so I bought one off eBay painted it and managed to fit it without removing bumper they do just push inwards and then fall down behind bumper just disconnect it and swap you can access it lying down behind car and squeezing hand in and over exhaust.
All the sensors were now working but still no sound so obviously the bleeper itself must have failed also there seemed to be a lot of confusion as to where the bleeper is but I can confirm the bleeper is in the middle of the melal rear shelf near the subwoofer 2 plastic screws hold it to the metal shelf I cut these off from inside the boot and then pushed it up you can feel it but cannot disconnect it I then took the grill off the subwoofer from inside the car it just pops out you can then access the bleeper easily there is a hole just above it factory cut pull out the electrical plug and swap for new reattach with a couple of trim clips it is held nice and tight and bingo everything worked a treat . Hope this helps anyone who is having trouble with the reverse sensors.