I have a Mac Mini 4,1. I get no sound at all except when I play through the internal speaker. Currently sound is supposed to go through the HDMI cable to the TV. I can see the HDMI output in the sound settings, but I get no love from the speakers. I've double checked the connection as sound works just fine in OSX. This is my first stab at using Ubuntu so ELI5 please.
I have a MacMini as well, same 12.04 haven't solved the issue but have a workaround.
Whenever the monitor/TV is not on during the boot process the sound doesn't work. I normally reboot or put the computer into sleep mode and wake it up again. The sound then works. Mine works with OSX as well but with Ubuntu I need to go through this odd workaround.