When I first install Ubuntu I couldn't get any sounds out of my speakers (OK that's was a stupid way to put it ;-). When I tried playing an audio or video, Totem keep playing them but no sounds and it was basically the same with every application. So what can you do to fix it?.
First of all, if the real reason behind this is a bad sound card or a bad driver, then there is nothing this method can do for you... (sorry about that) but still it is worth giving a try because it only takes 8 seconds I promise!.
To try to fix this (I assume you're using Gnome), from the main menu, go to
System -> Preferences -> Sound
That should bring a graphical audio mixer in Gnome and from that window click on the ...
*. "Output" tab (see the screen-shot below).
|Click on "Analog Output / No Amplifier" step 1|
|From the Drop-down menu, choose, "Analog Output / No Amplifier" (or "Analog Headphones / No Amplifier"), step 2.|
*. Then under "Connector", choose "Analog Output / No Amplifier",
if you're using headphones as the default speakers then choose "Analog Headphones / No Amplifier".
Now click "close" to close the window, now go ahead and try to play something if everything went well, you should be hearing some "music" :D.
But as said before, this won't fix all the errors concerning sound issues in Ubuntu, but still it is well worth an effort before starting to panic (yikes ;-)). If you have any problems, then throw me a comment.