Well before you resort to more extreme measures, check the Permissions for those apps that they're allowed to access the Microphone. Because of all the furor over privacy, by default a lot of apps need for us to specifically allow them access to various items (i.e. Camera, Microphone, Storage, etc.)
And don't be surprised if you need to dig a bit deeper in the Settings menu to find the Permissions options as it may in multiple locations (i.e. Settings >> Apps >> whichever app in question >> Permissions or some submenu in another menu. When you open Settings, type in microphone in the upper search bar.