There's a fair few ways of doing this, but the easiest is probably the following:
Install "pavucontrol" and "audacity" from ubuntu software centre or via terminal. (sudo aptitude install pavucontrol audacity
or sudo yum install pavucontrol audacity
).
Open up Audacity (from the menu or via a terminal with audacity
).
Select "pulse*" as recording device in Audacity.
Click Record Button.
Open PulseAudio Volume Control (Search For PulseAudio Volume Control in the menu or dash).
Select Recording Tab.
Now You Should see ALSA plug-in [audacity]. If you do not see ALSA plug-in [audacity], make sure you have clicked Record Button in audacity. You should be running recording in audacity while performing the next step.
Select "Monitor from *".
Now you should be able to record the sound you hear from speaker or headphones.