Heya,
we unfortunately won't change this anymore.
We tested the other default planes, they also can't volume control. The CJ4 can mute the frequencies, this is true, however the radio is sitting inside a different default framework that won't act the same as the 310s radio set. I know this is frustrating to hear, so sorry in advance.
You should however be able to just disable the COM2 frequency with vPilot dot commands. ".com 2" should enable/disable the channel.