Also, when you bring your kneepad up by using the ESC key, go into setup. There is a Roger Wilco button on that screen. Click it and make sure you have RW set to "Enabled". You can also set Aces High to start RW automatically when you start the game, if you wish.
When you close this RW setup screen, you should get a message in the game "Voice Settings Updated".
Some people have problems mapping push to talk to a joystick button. I've got a Sidewinder Precision USB stick, and it works fine for me, however. I've got my 4 button mapped to the "T" key in the Sidewinder Profile software.