I am not a Mac user, so I can't say what is wrong, but I would check a
few things. I would check the mixer in the sound applet to make sure
that everything looks okay. Then you can try this:

Open a terminal. type: gksu gedit [enter]

Navigate to and open this file: /etc/modprobe.d/alsa-base.conf
Add this line:
options snd-hda-intel model=imac24

Save and reboot. Let us know if it works.

Courtesy of: blog.costan.us/.../...-810-or-904-on-mac-mini.html