Step by step instructions.
Please Note: I assume no responsibility if you brick your unit. Following these instructions is at your own risk and peril.
I've made this as straightforward as I can, assuming a limited knowledge of computers, but really, if you don't know at all what you're doing, you probably shouldn't be doing this.
1. Get the testmode files from This Post.
2. Unzip that and copy the Testmode and MiTACAP folders onto an SD card
3. Grab the silence.wav file from the top of this thread
4. Make 4 copies of silence.wav and rename them so you have four files which are all silence, named: PSG1Hi.wav PSG1Low.wav PSG2Hi.wav PSG2Low.wav
5. Copy those files onto the SD card
6. Put the SD card into the Fxxx headunit and turn it on.
7. It will boot to test mode, with some info about copying stuff. Wait a few seconds then tap the X in the top right corner to get rid of that window.
8. You should now have a windows Start button in the lower left corner.
9. Use the Start menu to open windows Explorer. Do this twice so you have two Explorer windows going.
10. On one explorer window, navigate to My Flash Drive, Data, Sounds. You'll see 6 or 7 sound files in there. I *strongly* urge you to back them up.
11. On the other explorer window, navigate to the SD Card and find your four wav files.
12. Tap a wav file, tap the 'Edit' menu, tap 'Copy'.
13. Go back to the other explorer window, tap the 'Edit' menu, tap 'Paste'
14. Repeat for each file.
15. When done, just close the two explorer windows, then turn off the car.
16. Wait a few seconds, remove the SD card, and turn the car back on. Poke around in the AV menus, there should be no more keybeeps.