To use ARC both the receiver and TV have to have it.
After you connect the HDMI output of the receiver to the single HDMI input on the TV labeled ARC you will have to turn it on in the TV menu. You may also have to turn the TV speakers off. Check the TV manual.
You may also have to set up ARC in the Onkyo receiver. It may need to be assigned to an input as the audio source.