source: https://mozinc.wordpress.com/2017/07/06/mic-loopback-ubuntu-hear-my-sound-through-speakers/

https://askubuntu.com/questions/123798/how-to-hear-my-voice-in-speakers-with-a-mic

Here is a solution that I’ve tested with Pulse Audio on Ubuntu 12.04.

    Install PulseAudio Volume Control (pavucontrol)
    Now we will route your microphone to your speakers. Do this by running the following command:

    pactl load-module module-loopback latency_msec=1

    On the Recording tab of pavucontrol, you can show all streams (combobox at the bottom) and then configure which microphone (if you have more than one) should loopback into the built-in analog stereo

To stop it running, run:

pactl unload-module module-loopback