The volume element of the audio controls has no function as unfortunately P3D doesn't provide control over the volume of each source. In fact the audio selector logic is pretty basic due to those sim limitations.
The code should:
Enable COM2_TRANSMIT if the VHF 2 mic selector is pushed, otherwise enable COM1_TRANSMIT
Enable COM_RECEIVE_ALL if the VHF 2 volume is above zero, otherwise disable COM_RECEIVE_ALL (i.e. COM1 receive only)
Enable the FS avionics master if the AC emergency or DC essential busbar is powered and the avionics master switches are set to ON, otherwise it will be disabled
The easiest way to verify correct behaviour would be to add the default radio stack to the panel.cfg as you can then see exactly how the default radio is being controlled by the 146 avionics:
[Window Titles]
...
Window07=Radio Stack
...
[Window07]
Background_color=2,2,2
size_mm=157,270
position=8
visible=0
windowsize_ratio=0.8
ident=RADIO_STACK_PANEL
gauge00=Bendix_King_Radio!Bendix-King Radio Audio, 0,0
gauge01=Bendix_King_Radio!Bendix-King Radio Nav-Comm 1, 0,29
gauge02=Bendix_King_Radio!Bendix-King Radio Nav-Comm 2, 0,86
gauge03=Bendix_King_Radio!Bendix-King Radio DME, 0,142
gauge04=Bendix_King_Radio!Bendix-King Radio Xpndr, 0,179
gauge05=Bendix_King_Radio!Bendix-King Radio AP, 0,224
gauge06=Boeing747-400!Nav-GPS Switch, 0, 226, 20, 20