On Linux with PulseAudio installed.

To list all of our devices we can run this command:

pactl list sources

We will be using the Source #Number from the command above here. Replace the placeholder (<Source Number>) in the command below with it.

Recording one input:

ffmpeg -f pulse -i <Source Number> pulse.wav

Recording multiple inputs:

ffmpeg -f pulse -i <Source Number> -f pulse -i <Source Number> -map 0 -map 1 pulse.mp4

Note: To record desktop audio all you have to do is get the source id of the headphones/speakers the audio is playing to.

If you are on Windows, the command below should work for you or hopefully atleast get you in the right direction.

ffmpeg -f dshow -i audio="full audio device name" out.mp3

DirectShow Wiki For More Info: https://trac.ffmpeg.org/wiki/DirectShow