#get PulseAudio outputs which you can record from 
pacmd list-source-outputs|grep 'source:'|sort -u|grep output

#get PulseAudio inputs (microphones) which you can record from 
pacmd list-source-outputs|grep 'source:'|sort -u|grep input

#Record multiple sources to separate files
ffmpeg -f pulse -i 6 -map '0' 0.mp3 -f pulse -i 3 -map '1' 1.mp3

#Record multiple sources to a single file
ffmpeg -f pulse -i 25 -f pulse -i 2 -filter_complex "[0][1] amix [a]" -map "[a]" -threads 2 mix.mp3