Before messing around with uninstalling and installing various sound programs to fix a problem, check your volume levels. It is often the case that a muted channel is the problem, even though the description may not sound immediately relevant. In this regard, muted Surround or Center channels are common culprits. So unmute and raise the volume of one channel at a time and check whether sound is then produced by a running sound application e.g. Rhythmbox.
You can access the volume levels for the various channels from the command line or the GUI.
1.From command line: Open a terminal window and launch alsamixer. Then unmute as described , above.
*Note: Often you need to specify the card you want to control, do that by launching alsamixer -c0 or alsamixer -c1 etc, where the number after -c corresponds to your soundcard's index. To know what indexes your soundcards have, see below (under "Checking sound device assignment").
2. From GUI: This can be done by running something like gnome-alsamixer from a terminal window or navigating to your your Volume Control. Then unmute as described above. Also check that your switches are set correctly - for instance that if you use the analog output the analog switch is set ON or that the digital or S/PDIF switch is set OFF. You can select which tracks/switches are visible in Volume Control (see above) under Edit->Preferences.
* Note: These controls have equivalent functionality to alsamixer but with a graphical interface. Checking or unchecking devices in the these accomplishes the exact same thing as unmuting or muting devices, respectively, in alsamixer.
*Starting with Ubuntu 7.04, the sound volume control is also accessible from Menu->Sound&Video->Volume Control. It may be hidden in your menus but can be shown by going to System > Preferences > Main Menu and under Applications > Sound&Video ticking Volume Control.