I had some strange issues with the built-in microphone on a Lenovo IdeaPad 310-15IKB laptop under Linux Mint 19.1. For example, it was not working with Skype, but was working with Gnome Sound Recorder. The microphone input volume indicator in Sound configuration window was also acting strangely and seemed mostly insensitive to sounds (but reacted to tapping on the microphone opening).
I believe that the built-in microphone is mono because it has only one opening (on the top of the laptop screen, to the left from the camera), so I would like to configure it as mono and not to have to manually disable one of the two "input channels." Moreover, changing the input volume in system's Sound configuration window breaks this workaround, and one of the "channels" has to be disabled again through the PulseAudio Volume Control.
Even if for some strange reason this microphone is stereo despite having only one opening, I imaging it will be better to configure it to work as mono.