So the next rework gave it the previously described changes + a 2:1 makeup gain on the summing amplifier to recover some of the gain lost by not cascading the microphone preamp.  There's also a voltage follower to try to aid powering the phones.  The mute switch got some new resistors to try to soften the pops.

1 of the pots was rediscovered to be defective after the 1st discovery, 10 years ago.  

There's now a long delay on startup where the microphone doesn't mute.  There should be another resistor limiting the base current.

There's not much gain before distortion, but the independent gain is a lot easier than cascaded gain.  Not sure if it's trouble powering the headphones or the limit of the LM324.  

A better op-amp would be the next step.  The next circuit would have to be surface mount.  While through hole is visually appealing, it takes too much room.  The entire surface mount circuit would fit on the underside of the control panel.  

It needs some holes on the side & a zip tie to manage the cables.

Despite its tactile appeal, the audio quality & real estate is a disaster compared to what a soundblaster 16 could do 30 years ago.  The microphone & speaker could be fed into any modern sound card, mixed in 16 bit PCM, & sent to any speakers with a few lines of software.  There might be a single analog circuit to split the microphone between the phone & confuser.  The confuser only has a single analog output with 2 channels.  Maybe an adapter could split the output into a mono headphone & a mono microphone output.