asterisk users - May 2007 - Play sound file on keypress (bridged call)

Hey People,
Sorry if this has been covered before but for the life of me I couldn't 
find the answer.
So if someone could just point me in the right direction that would be 
greatly appreciated.

Is it possible to set up in the dial plan to assign keypad numbers a 
sound file on a bridge call.
I know that you can do it if the user just rings into the dial plan but 
was wondering if anyone knows how to do that on a bridged call?

So for example:
I have just connected two callers together using the bridge function in 
the dial plan. I would now like to have when caller one presses keypad 5 
that Asterisk will play the preset example.mp3 file for the number 5 on 
that call.

Thank you for your time.

Kind regards,
Allan.