again, i no longer know where 'preferences, general setting' is. 12.04. is a different setup with different terms. there are no general settings in System Settingsgo to prefernces, general setting, choose all setting and in the general setting (first option) and check "prevent power management during playback"
that will do the trick.
ahh its the ppa! i will let caffeine go then.caffeine is a solution but from my experience it not work quite well.
not to mention that you need to add a ppa, this i am not recommending!