Some devices automatically adjust your microphone input volume based on a few factors. This can usually be solved by going into the properties of your audio input device and turning the settings up.
A user may mute themselves so that nobody can hear them, and you can also mute them yourself. If a user mutes themselves, the microphone icon on their camera will turn yellow with a line through it. If you mute them, the microphone icon will turn red with a line through.