I get sound out of all 7 speakers. A couple of examples playing a 7.1 source (m2ts file channel check from Hunger Games):
All Channel: 7 Speakers
THX Cinema: 7 speakers. However the rear surround speakers share the sound with the side surround speakers (e.g., the same sound that comes out of RIght Surround comes out of Right Rear Surround and vice versa).
Pure Audio: 5 speakers. Rear surround speakers silent, the sound that's intended for them comes out of the side speakers.
So this makes me think it's something up with the input signal since it's being read and treated as 5.1.