In the 'Recording' tab of PulseAudio Volume Control, drag the volume sliders so that the recording level in Audacity's Recording Meter is to left of 0 dB (-6 dB is a good level to aim for).Launch the application that you wish to record and begin playback.In the 'Capture from' dropdown, select the 'Monitor' option of the playback device used by the application you want to record from, as shown below:.Open PulseAudio Volume Control and choose the 'Recording' tab.Ensure that 'Software Playthrough' is not enabled in the Transport Menu then left-click on the Recording Meter to turn monitoring on.Audacity's playback device may be set to pulse or to any available audio interface.On systems where PulseAudio is the default sound system the recording device may be set to default.In Audacity's Device Toolbar, set the Host to ALSA and the recording device to pulse. This is usually available in the distribution's repository. If not already installed, add PulseAudio Volume Control (pavucontrol) to your system.

Setting PulseAudio Volume Control to capture from the Monitor device lets Audacity record computer playback when its input device is set to pulse. PulseAudio Volume Control provides a 'Monitor' device which listens for the audio output of other applications such as Firefox or Rhythmbox.

PulseAudio operates as a proxy between sound applications and the audio hardware (usually via ALSA). It is the default audio device on many Linux distributions including Ubuntu, Linux Mint and many Debian/Ubuntu-based distributions.

PulseAudio is a sound server providing professional audio features such as mixing and input/output routing.

Audacity recording manual