Let us suppose that you want to record the audio playback of your computer without any external recording system.

A poor man's solution would be to plug a microphone in the audio input jack and record from it, say with the sound editor Audacity. Beside the bad quality of the sound recording, a drawback of this approach is that it is going to pick up any noise in the room, unless you have two separate input (mic) and output (headpĥone) jacks on your computer, in which case you can use male-male jack cable to connect both.

Here is a better solution, using the Audacity and PulseAudio Volume Control:

1. Launch both audacity and pavucontrol.

2. In Audacity.

3. Open the menu Edit/Preferences:

   • In the Devices Tab,
     • select 'ALSA' in Interface/Host.
     • select 'pulse' in Playback/Device.
   • In the Recording tab, make sure that Software playthrough of input is not selected.
   • Close the Preferences window and start recording (Shift+R)

4. In PulseAudio Volume Control:

5. In the Configuration tab, under Built-in audio, select `Analog Stereo Duplex``

6. In the Recording Tab, showing 'all streams', you should see the Audacity stream (Audacity must be in recording mode!), and Audacity should be recording the audio playback of the computer.
   

Remark: In Ubuntu 20.04, I am using versions 2.3.3 of audacity and version 4.0 of pavucontrol.

Recommended reading: How to use Pulse to manage sound on Linux

Other possible approaches

• Use alsamixer to enable the loopback device ('y':disable 'e': enable).

• Use alsaloop

• Use the snd-aloop kernel module to set up a virtual audio loopback device (see https://sysplay.in/blog/linux/2019/06/playing-with-alsa-loopback-devices/)

  modprobe snd-aloop arecord -f cd -D loop > recording.wav aplay -f cd -D hw:0,0 recording.wav

pactl load-module module-null-sink sink_name=MySink PULSE_SINK=MySink parec -v -d MySink.monitor --file-format=wav recording.wav

https://unix.stackexchange.com/questions/127567/recording-audio-from-web-based-audio-player-using-alsa-loop-device