I think we want to check that I also have an optical cable from the HDTV back to the Onkyo. ARC is hit and miss so I always add that last connection.
I have recently purchased a LG 42LM660T tv. I have a virginmedia box and a playstation 3 that are connected to a Onkyo HTX-22HDX Home theatre system through HDMI and in turn the Onkyo is connected to the TV through HDMI Arc. Everything to working brilliantly picture and sound wise (on virginbox and Playstation 3) until I select the Smart TV function on the tv. I can get picture ok but I cannot get sound, the sound I get is still whatever channel is on the virginmedia box. Any help would be much appreciated.