I have often wondered about this myself. I like to use headphones... I wonder if you could use a stereo recorder, and put the "speaker out" (low impedance) to one side, and an external mic (high impedance) to the other. Getting the audio to balance would be a trick.

We need a small portable, two channel mixer with low and high inputs and a balance control.

Otherwise you have to use an external speaker, and just pick it and your voice up over the mic.

Bill W1PA