I upgraded to Ubuntu 13.10 on my media box, a Zotac Zbox, , ION GPU, with the latest official nvidia drivers. After that I couldn’t get the sound over HDMI to work. Panic! After a LOT of trouble shooting I found the error – the spdif1 channel was muted in alsamixer.
So to fix this, open the trusty terminal, enter alsamixer, and lookout for the spdif1 which probaly says MM which means muted. Press M to unmute/mute a channel and raise the volume staple. You hopefully now have sound over HDMI again!
(not my pic, found it on the net, thanks to whoever took it)