While I use an ffmpeg laucher that has various scripts (in the quicklists) for screen capture, your command isn't that different.
Ex. of one with audio-
What you do need to is do a one time setup of pulseaudio, there's likely a thread about that may describe how in detail & any variance for 'what you hear' vs. a mic.
gnome-terminal --geometry=20X4 -x ffmpeg -f alsa -i pulse -f x11grab -r 24 -s 1280x800 -i :0.0 -c:v libx264 -preset ultrafast -crf 0 -c:a pcm_s16le ./Videos/screens/$NOW.mkv
In any event it needs to be done in pavucontrol, not sound setting. Basically just start ffmpeg recording with sound, then open pavucontrol > recordings & switch to the 'monitor of ...' option.
You only need to do this once, from then on 'monitor of ..' will be auto-selected for ffmpeg ( at least the real FFmpeg
(pavucontrol is not default installed on Ubuntu