Walking with the desire to do karaoke, I found that there is no option to hear my own microphone, neither in Gnome nor in KDE. But the command line could not fail; just load the PulseAudio loop module:

pactl load-module module-loopback

And to disable it:

pactl unload-module `pactl list short |perl -ne 'print $1 and exit if /^(d+)s+.*module-loopback/;'`

Or in a handy script:

pulseaudio-loopback.sh

#!/bin/bash

if pactl list short|grep module-loopback 1> /dev/null
then
    echo Desactivando el bucle de audio
    pactl unload-module `pactl list short |perl -ne 'print $1 and exit if /^(d+)s+module-loopback/;'`
else
   echo Activando el bucle de audio para el dispositivo de entrada preferido
   pactl load-module module-loopback
fi

 Sound quality leaves to be desired, nevertheless, but that's another roll.