Click the inner knob to go to nav freq mode, and turn on the volume. Then the NAV radio is powered. If that does not working, try to plug in the headset.
Might not be quite the right way, but because we support 3 different units, plus the audio simulation with the headset, it is a small trade off, to have everything working.
Btw, I've asked the author to add a couple datarefs, so I can know the state of various functions on the unit, but he was not willing to help. For example, when you go into nav freq mode, the volume knob controls the nav 1 volume, but after a few seconds returns to com mode. I have to track your clicks and run timers every time you click this knob to figure out what the volume knob is doing.