Way back in the 1950's, I wired a headphone jack into the speaker output on my radio.
Bear in mind that the headphone was high impedance, several K ohm.
It worked quite well, and the speakers were rated at 3.2 Ohm mostly back then.
Presently I have a bunch of 600 ohm headsets that came out of a school, and I may just try connecting to an 8 ohm output, such as the kind used in BITX40's, and more. These are headsets that will be reasonably comfortable to wear.
Many mikes are rated at 600 ohm, usually dynamic.
I have a mike from a Midland that is rated at 600 ohm, but I am wondering if it is an electret mike, as it tests as being an open circuit.
That or the element is blown.
With speaker, headphones and mikes, they will work with a mismatch most of the time.
Won't hurt to try using it as an electret if it tests as an open element