I cannot comment about specifically your set up, but here is something that I have observed.
I have a slingbox pro, streaming to slingplayer (on a pc). The audio and video are almost perfectly in sync.
I also stream from the slingbox pro to a slingcatcher. The output of the slingcatcher is connected to a Sony Bravia TV using HDMI cable, and the audio is also connected from the slingcatcher to a hi-fi system, using a digital coax cable.
For the audio/video sync when viewed and listened to on the TV, they are in sync. At least as good as a normal digital TV transmission. But the audio played via the hi-fi system is very slightly in advance of the sound coming out of the TV speakers. About 0.1 second or less (difficult to judge).
Of course this sounds like an echo if both hi-fi and TV speakers are used at the same time.
But, in my case I would not consider this a fault. It would seem to me that the data stream arriving at the slingcatcher (from the slingbox pro) is fine, only the processing time of the audio and video streams in the slingcatcher and the Sony TV is slightly different (depending on using HDMI cable, or using digital audio feed).
By the same reasoning, I would say that any device that needs to process and display both video and audio needs to process both at exactly the same speed and present them to the display device at the same instant in time. I believe that many devices have a setting (somewhere) which can be used to delay the audio (or video) slightly, such that they arrive in sync.
In your case, I wonder if this is a similar thing?