How to connect SONY IF wireless headphones to ONKYO reliever

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.
 

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.