Although I do have the case and various mounts, most commonly I shoot handheld videos without the casing. In doing so, I have discovered that sometimes I inadvertently cover up the microphone port with my fingers which muffles the sound. Given that the GoPro HERO7 Black is waterproof, I had always assumed incorrectly that the microphone was using the same port as the speakers on the bottom of the camera.
The owner's manual does not show the location of the microphone port, so I searched Google. All of the search results led me to external microphone products or general reviews of the camera. After doing some of my own testing, I discovered that there is a separate microphone port on the side of the camera—it's the tiny hole above the power button, as shown in the upper left portion of this image:
So for those of you who shoot handheld videos with the GoPro HERO7 Black and were wondering the same thing, I hope this helps you too.
