There is about a 70ms latency between Sonos Line-In and output from a Sonos amplifier or player -- even if the Line-In is on the same box that is connected to speakers. Sonos deliberately does this in order to keep all of the players time aligned across the network. Also, sound travels at about 1 foot per millisecond. Even if a speaker that is 40 feet away is time aligned with the speaker near you, the more distant speaker will seem "late". If you then move close to the more distant speaker, the original speaker will seem "late". You can use electronic delays or clever positioning to align the apparent source of the sound, but this alignment is valid for a very small region.

Typically, the latency associated with 7.1 processing is less than the 70ms Sonos latency and the Sonos associated Zone2 output is perceived as "late". A straight forward electronic delay in Zone2 does not work in this situation because the delay would need to be negative (predicting future output).

Depending on the signal processing scheme used inside the receiver, tinkering with the receiver's lipsync may help. You may also have some relief if you tinker with the speaker "distance" associated with each of the 7.1 speakers. (note that this may seem counter intuitive at first because the closer speaker must be delayed in order to align with the more distant speaker)

chris-L5S: Start by claiming that the front speakers are much more distant than the rear speakers. This may injure the home theater experience a little, but the rear speakers will align better with the Sonos output in the bar. Note, however, this scheme will suddenly break down if you switch the receiver to a "direct" mode.