iMac not longer have 5.1 output

I have mid-2011 imac. I use VLC player to play my .MKV movie files. The files contains 5.1 audio file in them. I hook iMac using HDMI for video and Optical audio output to my Lifestyle V28. When I first bought the iMac (late july) I remember seeing the bose system display "5.1 DTS". but now it always display "2.1". I just want to get the best video (already have it) and sound when i am playing a movie.

anyone know why this is happening? or recommends a different MKV player?

Thank you!