I am using now 13.04 and sound 5.1 doesn't work well, only the 2 front speakers work, in Ubuntu 12.10 work all good all speakers play. I am using the USB sound card sweex SC016.
1. Two active soundcards
There are two active sound cards in a computer: one on the motherboard and a separate PCI sound card. Or a built-in old-fashioned phone modem: that has it's own sound chip, for making calling sounds.
In certain rare cases, Ubuntu has a problem with the presence of multiple active sound cards. Then it's therefore best, to have only one active sound card.
Solution: disable the on-board card in the BIOS. After you reboot your computer, the sound will come entirely through the PCI card.
If you can't find the disabling option of the on-board sound card in the BIOS, search the Internet for the manual for your motherboard.
2. The volume may not be right or too few sound channels are opened.
A. Install pavucontrol by means of Ubuntu Software Center.
B. Now start pavucontrol. Check the position of the sound channel sliders.
3. Install alsamixer and try this sudo alsa force-reload .
4. If none of the above work,try this. Just open the terminal and copy these commands.
When you finish, restart your system. If sound still missing from your system, you can check this article we published some months ago. If you have something to add, please do by commenting below.
Some external sound cards are not detected properly by PulseAudio the first time and you only get the stereo profiles to choose from. In that situation killing PulseAudio with