I did some investigating on a uverse forum and found my answer:

Quote:
Currently, the software on the AT&T set-top boxes can only pass 2.0 channel audio via HDMI. Supposedly, the box hardware can handle sending 5.1, but the software cannot do that yet. AT&T may issue a software update at some point in the future that addresses this.

In the meantime, you can use the optical audio out to get discrete 5.1, or you can run the HDMI 2.0 audio through a Dolby Pro Logic II decoder to get 5.1 sound (the audio on the HDMI is a Dolby Pro Logic II compatible matrix-encoded downmix).
That jives with what I have experienced. The PLII 5.1 actually sounds really nice though.
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Main system:
Integra dhc 9.9
Threshold SA/4e pure class A
Emotiva XPA-1 (2), XPA-5
(2) Threshold S200's
Thiel 3.6 main speakers
(2)Velodyne F1500r subs
Polk RTI28 surrounds
B&W HTM center
OPPO BDP-83 universal player
Samsung HLT6187 led DLP