If you find yourself with no sound when using vlc, rhythmbox or whatever (except mp3blaster, yeah!) after an apt-get upgrade in Ubuntu 9.10 «Karmic Koala», then the problem MIGHT BE that the default sound output changed to the HDMI one.
Just right click on your speakers icon, choose «Sound Preferences», go to the Output tab and make sure the correct output device is chosen. In my case, I have a HD48x0 audio Digital Stereo (HDMI) output, and a «Internal Audio Analog Stereo». Make sure you choose the one that is currently connected to a speaker set. For some reason, after an upgrade and reboot, my default output got changed to HDMI.
Switching it back to the analog stereo one solved it.
Now, I really want to go, get the damn hd cable and plug it to my stereo 😛