Comment 28 for bug 147087

I consulted /usr/share/doc/alsa-base/driver/ALSA-Configuration.txt.gz and a possible fix is to try the following:

edit /etc/modprobe.d/alsa-base, e.g.:

gksu gedit /etc/modprobe.d/alsa-base

and at the end add add:

options snd-hda-intel model=imac-intel

or:

options snd-hda-intel model=imac-intel-20

I am not entirely sure which of the two apply for your machine, I expect it is the latter, but I suggest trying both out one at a time (e.g. make the change and reboot and check the sound, then try the next option in the same way).

This is a required workaround for the sound device having PCI Id 8086:284b. There is an existing wiki page describing this device: https://wiki.ubuntu.com/Gutsy_Intel_HD_Audio_Controller, but it does not have any suitable workarounds listed for your machine.

Please let me know if this gets any results.