Easily record the sounds heard through system speakers on Linux with Pulseaudio and Audacity.
apt install audacity pavucontrol
To record “what you hear” in Audacity: “click to monitor” (right of microphone in main Audacity screen). Start Pulseaudio advanced configuration tool
Under pavucontrol Recording tab input, select “Monitor of Built-in XXX” where
XXX is the playback device you wish to monitor.
This loopback audio from pavucontrol allows recording what you hear in Linux using Audacity and PulseAudio.