Audacity is only recording silence

Causes

Your microphone gain may be reduced. To fix this, see: setting-recording-levels-and-playback-levels.md
You may be attempting to record a Bluetooth device at an unsupported sample rate. To fix this, see audacity-records-silence-when-using-a-bluetooth-headset-as-a-recording-device.md
You may be attempting to record a device which doesn't produce any sound. To fix this,
- ensure that all parts of the device are connected properly (cables, microphone, interface, etc),
- that microphones in need of phantom power are provided with that,
- that no mute switches on any devices are turned on,
- that no faders are turned down, and
- that an input signal is being produced by the source.