CD adapter for headsets

Often when browsing catalogs I've seen wire adapters that allow a
music source (CD/MP3 player) to be plugged into your headset. I'm not
talking about electronic devices such as PS Engineering's "The Muse",
but just the basic physical wire harness.

Does anyone know if these are simply an audio jack wired directly to
the audio pins of the headset plug, or is there some other "stuff"
(ie: impedance matching) going on in there.

Chris