It is an issue with Ubuntu 14.04 beta. I cannot listen any music or videos because the system showed no output device in the audio settings or just some dummy output. There is one way to fix that is to make sure my user is in the pulse and pulse-access group which provide access to audio driver.
To see which groups I am in:
$ sudo grep trinh /etc/group
Add myself to pulse and pulse-access group:
$ sudo usermod -a -G pulse,pulse-access trinh
And now I can hear the sound again!