i'm using an 07' iMac Aluminum...and i've found that ALSA has botched the sound profile of the RealTek ALC 889A
...if this is indeed the problem... the best way to correct it is to manually specify a sound model that most closely resembles the 889A ... the 885...(Macbook Pro Revision 3)
open your fav text editor to a blank file named 'sound' in /etc/modprobe.d...you can do this in Gnome by:
su -c 'gedit /etc/modprobe.d/sound'
and insert the following into the empty file and save:
alias snd-card-0 snd-hda-intel
alias sound-slot-0 snd-hda-intel
options snd-hda-intel model=mbp3
reboot and you should be good to go...
hope this helps... let me know.