My onkyo has audio out listed under cassette #2 which is also input/output for the equalizer (yeah, it's 23 years old), the only others would be the audio out for the secondary speakers or the 1/4 headphone jack in the front.
What are exact models of both headphone and Onkyo?
ONKYO Family Room Receiver Model TX_SR707
Sony Cordless Headphones Model IF-254R
Did get an audio signal with plugged into red and white "OUT" outlets, but not "Audio Out" as described on website. Only worked within 5 ft though.
You get audio out to the RF transmitter from the Zone 2 line output. That output has it's own volume & input controls so you can increase the input to the transmitter if needed.
If the range of the headphones is 5' you need to test the transmitter by connecting another source (like your phone) to it.