5.1 sound works through Beamer, not AirPlay display

Hi there,

I have a MacBook Pro (early 2011; 10.10.4) and an Apple Tv (A1469; software 7.2 7512) connected through optical to a surround system.

When I use Beamer (2.1.3 21008) to play a movie (with, for example, 6 channels and AC3) the surround sound works perfectly fine. However, when I use Airplay Display to mirror or extend my desktop and I play that same movie on QuickTime 10.4 (833.6) or VLC 2.2.1, sound is only stereo. Surround sound, obviously doesn't work either when I try to play 5.1 content on the web (i.e. Netflix, Filmin...).

Could you guys help me?

Thanks a lot