[nthnclrk]

Without further investigation, I pose a potentially naive question:

What evidence is there that this behaviour of the internal mic is the same when the Audio preferences pane is not open?

Could it be that the mic is automatically made 'live' when system preferences or that particular pref pane is opened?

[bradleyland]

Even if that's the case, it's not really relevant. If the concern is that an attacker will listen your mic, the only 100% solution is to physically extricate the mic from your laptop. If an attacker is able to install software (with administrative privilege) on your computer, they can do anything you can do, including re-configure your audio devices and mic volume levels.