Instead of assigning "Radio 5" assign "voice transmit channel". Rudder and rudder trim can be set with keys, buttons, or analog control.
If you assign "rudder left/right" under "global" to buttons and have rudder assigned to an axis then the buttons won't do anything.
Make sure you have "trim rudder left/right" under "flight" assigned to your trim buttons. That's what Delirium was suggesting.