This is a note to myself.
Two packages are required: gtk-recordmydesktop and pavucontrol
In gtk-recordmydesktop, under advanced settings, set the audio device to “pulse” (instead of “DEFAULT”)
Once you start a recording, in pavucontrol there will be an entry saying something along the lines of “ALSA plug-in [recordmydesktop]: Also Capture from ….“. Set that to “Monitor of Built-in…” and the audio recorded will now be the same as your output audio, and not your microphone.
pavucontrol should remember this setting for the next time you use recordmydesktop…. maybe? But test that first with a short video of, say, a youtube clip to make sure, because recordmydesktop doesn’t give any indication of what audio it is recording.