Your symptoms suggest the player is sending stereo, not multichannel. Is the audio output priority set to HDMI? And is the HDMI PCM output set to stereo or multichannel? (I don't recall the exact settings and terms used on the Sony 550. Haven't looked at that manual in awhile. But, they're something like that.)
Stereo output can sometimes happen, regardless of the player output settings, because of the HDMI audio handshake with the TV. TVs are 2 channel devices and the set may be telling the player to send stereo. Receivers often have a setting to disable the HDMI audio output, which would solve that problem. Or, as a test, you can simply unplug the HDMI connection to the TV and see if you start getting multichannel audio.