its on rosstechs page under B5 A4:
35-Central Locks: -- KWP-1281
VAG-COM Works fine (note: pre-'97 A4's may not have this module).
Recode locking options
[Select]
[35 - Cent. Locks]
[Recode - 07]
Enter the coding below
[Do It!]
Write down the existing coding for future reference.
Add the following options to your coding number: +64 Windows up, sunroof close, and windows down with remote.
-04 One touch unlock of all 4 doors and trunk
-1024 Remove horn honk when arming
+32 Auto lock/unlock of doors (Doors lock at 15 mph and unlock when key is removed)
So if your car doesnt currently honk, then you'd add 1024 to the existing coding in that channel and save it.
Done this on craigs S4 and it worked just fine. Its a bit of a silly noise, but its instant audiable notification that the car has locked successfully. If a door is left open or somethings not right it doesnt hoot, so you know theres something odd. Whereas just having the flashing indicators is easily missed.
Early A4's did it by default, and it cant be disabled. For some reason on the later cars (after 97ish) they disabled it using that coding value.