Create a dummy microphone with pulseaudio

$ pactl load-module module-null-source && pactl set-default-source source.null

This is helpful in situations where a program fails to function properly without a microphone, even though one isn’t actually needed, and you don’t have any microphone hardware connected.

Update Oct 2021: In the meantime Fedora has switched to PipeWire. Unfortunately, pipewire-pulse does not support module-null-source. The Migrate-PulseAudio wikipage has a note to do the same with pw-cli, but this did not worked for me. Fortunately the Virtual-Devices wikipage has a working solution:

$ pactl load-module module-null-sink media.class=Audio/Source/Virtual sink_name="Null Source"