Q: How to connect my iMac to AV receiver?

To do this the right way, get an AppleTV ($59 refurbished from Apple's online store), set up the AppleTV and  connect it to the AppleTV's Optical out to the Receiver & HDMI out to your TV. Then set up Home Sharing in iTunes on your iMac and finally wireless stream movies to the big screen TV from the AppleTV. Anything else is a kluge and won't work as well as  you would like. If you have a 2011 or later iMac  you can also use the AppleTV to mirror the iMac's content to the BigScreen TV.

http://store.apple.com/us/product/FD199LL/A/refurbished-apple-tv

Learn more about Home Sharing - Apple Support

About AirPlay Mirroring in OS X - Apple Support

get a cable which connect to the headsets digital part and connect it to the speaker systems

tigerrr_ wrote:

 

I don't have a separate TV. I can only watch movies on my iMac. Is this still possible.

 

i Highly appreciate your help!

Yes, however the Mac does not have a TV receiver, you need a their party device from Elgato to connect from your TV source to the iMac.

https://www.elgato.com/en/eyetv

Then it is VERY easy, if your receiver has optical input you need this cable:

http://www.amazon.com/FosPower®-Toslink-Digital-Connectors-Strain-Relief/dp/B00T 8HWVEY/ref=sr_1_2?ie=UTF8&qid=1427659373&sr=8-2&keywords=minijack+to+optical

The receiver doe not need Bluetooth or Airplay, all it needs is the optical input to stream music from the iMac via the AppleTV. I haven't tried to stream music wirelesses from our iPhone or iPad however  you don't really need to. If you have music in iTunes for $25/year you can buy Apples Match (Google it) and then your AppleTV can download your music library wirelessly via the Cloud.