As you have a serial number then someone like
Can provide a code for a small fee
If you check the handbook first though as often the radio code is on a sticker inside the front cover or inside the audio manual.
It would also be worth contacting the local dealership as they often carry the codes for recently built cars ( last 10 years)
and its worth trying Audi customer care
If you tell them your Vehicle ID number and the radio serial number they will ask you some questions to verify the vehicle is registered to you and then they should provide you with the code for FREE. This typically takes a couple of days.
It is illegal for anyone to access the radio code through the Audi data base without verifying the customer, vehicle, and radio serial numbers. If someone were to provide that service for you here online, they would be risking their employment as well as legal ramifications.
I realize this is an inconvenience, but it is the main security system that keeps thieves from breaking into your vehicle and stealing the radio
its also worth trying the last 4 digits of the serial number as often this is the security code and sometimes the code is on a sticker in the spare wheel well in the trunk
then to enter the code
Turn on radio. The radio display shows "SAFE".
Press the P.Scan and RBDS buttons simultaneously and hold until "1000" shows on the display.
Release the buttons. Do NOT press these buttons again, otherwise the radio will consider "1000" to be the code.
Use Preset buttons 1 to 4 to enter the four-digit code number from your radio card. Preset button 1 corresponds to the first digit, Preset button 2 corresponds to the second digit, etc. Each press of the button increments the number by 1.
Once the code shown on the display is correct, press the P.Scan and RBDS buttons simultaneously and hold until the word "SAFE" appears in the display, then release the buttons.
The radio should unlock and be usable shortly afterwards.
If an incorrect code is entered, the word "SAFE" appears, first flashing and then continuously. You can repeat the above steps to unlock the radio once more, the number of attempts will be shown in the display. If an incorrect code is entered again, you radio will be locked for about one hour.
After one hour - the radio must stay on and the key must remain in the ignition lock - if the display goes out, the you can repeat the above procedure to unlock again.
The cycle - two attempts, one hour lock-up - still applies.
If this doesn't work then its possible that the radio has been replaced at some point so the code in the book no longer applies