Yes, some audio delay isn't uncommon
It's another reason to grab a real receiver with adjustments for such audio delay issues. The great news is most of these features are now available even on the lower/budget priced HTIB units.
Definitely mute the built in TV audio since it will pale to what the Sony will sound like, even only w/ matrixed sound. I don't know too many people who *don't* mute the built in tv audio once they get proper surround set up in the room(?)