Install PulseAudio volume control
Start both pavucontrol and Sound Recorder, and start recording sound with Sound Recorder. Go to the Recording tab in pavucontrol, and make sure "Applications" is selected in the drop down menu in the bottom right corner. Sound Recorder should now show up. In the drop down menu next to Sound Control, choose "monitor of build-in audio analog stereo". All internal audio, including audio from the internet, will now be recorded by Sound Recorder.
sudo apt-get install pavucontrol