The simple answer:  It's not!  

For cameras with display screens, you will notice two sets of icons on screen:  

1.  The set of smaller icons across the top of the screen are "status" buttons referring to the current status of the camera.  From the left, the first indicator is the recording status.  

2.  The set of larger icons on the bottom of the screen can be thought of as "action" buttons, and refer to what happens when you press the corresponding physical button beneath the screen, to activate the action for that icon.  (Note the SC 400 is a touch screen, so there are no physical buttons.)  

SC 200, 201, and 400 cameras:  

With these, you will see the icon for the Microphone on the bottom of the screen with a red slash through it by default while recording, this is normal and means your recordings will have audio on them.  Because it is an action button, when you press it you are muting the camera, and it will change to a microphone icon with no red slash; pressing it again will unmute the camera.  

SC 220C and 250R cameras:  

The same concept of status vs. action icons will apply, however the microphone icon at the bottom of the screen has been replaced with a shield and "Privacy Mode" icon that serves the same function as the previous microphone icon.  When pressing the shield icon you will have option to disable the microphone as well as a cabin view camera, if attached.  When disabled, you see the small microphone icon at the top of the screen with the red slash through it.  

Bottom line:

If this is confusing, consider the top where the status icons are.  When the TOP microphone icon has a red slash through it, the recording will have NO audio.  When the TOP microphone appears with no red slash, you WILL have audio on your recordings.  

The pictures below show the TOP status icons for various models, both reflecting that microphone is enabled and recordings will have audio.