Currently Being ModeratedNov 27, 2012 11:00 AM (in response to mahmon)
I have the same problem with my 27" late 2009 iMac. I am unable to output surround sound throught the digital optical output using a toslink cable with an adapter. But what I do to output surround sound is use the minidisplay port. I connect a minidisplay port to HDMI cable to my iMac and the HDMI input on my reciever. Check out my post about outputting surround sound from your Mac.
I hope this helps.