Adding a headphone output socket to the bat listener has been on my list of things to do for far too long!
It turns out it's quite simple to do. Replacing the speaker with a 100 ohm to the left and right channels of a headphone jack socket. The output is mono, so the same output goes to both ears. A 33nf capacitor is added for a little bit of filtering.
The addition to the bat listener circuit diagram is shown below.
This was prototyped and worked really well. It's quite a loud output, so it is highly recommended to use a set of headphones with an in-line volume control.
This has been added to the schematic and I am currently adding a headphone jack socket to the next batch of bat listener PCBs.