2 RCAs are for connecting to an amplifier/speaker. They output around 1/5 volt which is insufficient to drive headphones directly. A small amplifier, with a volume control could drive almost any headphones - if it has a speaker connected, this would need to be disconnected, and phones connected in place, but via a resistor of 500 to 1000 ohms or so, to prevent the phones being overdriven. Do you have other audio gear that would accept an input from the RCAs, and may have a 'phones output?