I had a KDL-40NX723 which would intermittantly, yet reliably, loose sync with the external speakers. The TV would pop up after an hour of use to say that it had lost connection, and it would default to the internal speakers instead. The only way to get this back was to power off the speaker bar, wait an hour, then turn it back on again. Otherwise, the TV wouldn't find it in the SYNC menu.
Of course, I suspected cables, and tried others. I made sure that the speaker bar was in the right HDMI socket on the TV.
However, I have found that this symptom occurs more often (if perhaps exclusively) when I have my iMac plugged into another HDMI port on the TV as an external monitor. It's an Intel iMac using the DisplayPort output to HDMI.
Unplg this cable, and the TV seems to keep sync with the speakers. As far as I can tell.
I wanted to tell someone this, in the hope that a firmware fix would sort it, or at least to give others some help in diagnosing similar problems.