When you click on the input thumbnail, it is sent to Preview. If 'Automatically mix audio' is active for that input, then the audio will then be muted because auto-mix considers that audio should be muted on preview and unmuted when live. This happens even if the input is already live somewhere (in Mix 2, for example). Previewing it in Mix 1 will cut off the audio, which is not necessarily what you want. So use auto-mix with care.

Usually, an internal or external microphone sets itself to mute automatically due to the incorrect sound settings. Even if you haven't modified those settings, they might have been changed automatically after the installation of some updates or malware infection.