I bought a Sony X900e TV about a month and a half ago. Everything is great so far except the audio - I've noticed that occasionally it will switch from using the audio system connected to my receiver to internal TV speakers.
It will do this multiple times in a "viewing session" - it happens for about 3 seconds and then switches back to the receiver speaker system. It will be fine for another 5 minutes or so and then do it again, and repeat.
Does anyone else have this issue? Any solutions? FWIW, my receiver is a Sony STRDH770.
Do you use HDMI CEC to control the receiver? If not, try turning it off in your receiver. It may have a different name because of marketing. On the TV, it's called Bravia Sync so you may want to look in those menus as well and see if a setting might be causing this.