You are going to have to connect the TV audio output to an input on the Onkyo. Depending on what kind of audio output the TV has and what inputs the receiver has you may need an active converter. Use digital audio or HDMI-ARC as you first choice. Headphone or analog audio out next.
If the TV doesn't have any audio outputs then you could open the TV and connect a speaker to line level converter to the wires connecting the internal TV speakers. That would give you a variable analog audio output.