Please tell us the results of your tests. "Can't get 31313 to work" doesn't tell me anything. What did you get for the blink back of the setup code? Does the setup code persist when you press other device buttons?

And a basic question, when you assign 31313 to the Audio button, does it blink twice?

Like this-- to assign a asetup code:
Press Audio
Press and hold P until 2 blinks
Enter 31313
You should now see two blinks, indicating that the remote accepted the setup code. Does it?