I cant have surround sound from imac via

i have an imac late 2013, 27". i try to use my samsung 55f8000 tv as display for movies and for audio i use sony da 5400es av receiver. Imac is hooked to the av receiver via HDMI. In normal condition i have to have surround sound from my receiver. but its 2 channel stereo. İ try everything but no solution now. İ extend the display for watching movies on my tv. I change audio setting to the HDMI. I use VLC, i change audio setting to the HDMI too. But on the MIDI app i cant see 5+1 sign, its always showing 2 channel audio.

İ thing when the imac detect that its hooked with a tv, its change automaticly the audio to the 2 channel stereo. How to avoid this.

thank you for quick replay