I believe I’ve found the cause and it’s related to the issue I mentioned about the COM/MIC buttons in this other post.
While I have COM2 and COM2/MIC selected, the GNS430 volumen knob works correctly. As soon as COM1/MIC is active, having COM2 active at the same time, it doesn’t work. In that case only the GNS530 volume works.
Occasionally (not always, but it happens sometimes), when all four buttons are selected (COM1, COM1/MIC, COM2 and COM2/MIC), the ATIS selected on COM2 is heard duplicated, as if it were also tuned to COM1.