The fact the user has to explicitly enable the microphone at session startup is a security feature, to avoid that the microphone can be inadvertently left active and so a user sitting at the remote side can hear what you say (with an appropriate program). Presumably you know whether there can be somebody at the remote side, but NoMachine can’t know.
Di you consider this? Would a dialog showed at session startup improve things? Of course you would be able to select “Don’t show this dialog again” if the dialog annoys you.