First off, this idea is an extension of Tlogmer's half-baked "Stealth Headphones" idea (see link).
As described in "Stealth Headphones", tiny speakers are hidden/implanted/embedded in the user's ear canals. But in addition to a speaker in each ear, the user also has implanted in each ear canal a tiny microphone. And somewhere on the user's person is a small, high-density solid state storage device for holding the recorded audio. At the flip of a switch, tap of a button, thought of a key phrase, (etc), the system can be switched into capture or playback mode. In capture mode, the audio is recorded as it enters the ear, and in playback mode, the previously recorded audio is, you guessed it, played back into each ear.
Simple enough, but the implications are interesting -- just listen to a song/conversation/lecture/whatever once, and at any time after that you can play it back to yourself exactly as you originally heard it.
Another way to drive the RIAA up the wall is to listen to a song on someone else's CD five times, and then go to sleep and have a lucid dream where you turn on the dream-stereo and expect that song to come on. Even if the song is from wax cylinder, your brain will repair it and it will play better in a dream than reality. This "dream-stereo" option is the only one I have in order to listen to Phil Fearon & Galaxy's "What do I do?".
Problem is that human's brains are very good at filtering out extraneous background noise. Any microphone would pick up what's either loudest or closest, neither of which may be what you'd like to record. This kind of system would almost require the brain to "download" a filtered and enhanced version of the audio onto some kind of device; a process which humans can already do. It's called "memory."
thelumberjack: if you record exactly what is received, and play back exactly what is received, then there is no difference between the sound heard from the recording and the sound originally heard from an external source. So I don't see what your objection can be.
This "memory" of which you speak... it can't be a very good solution, or why do people keep (and use) collections of music?
pottedstu - for some reason it doesn't work like that. 'Frinstance, the ear will be able to filter out the inherent "echo" present in a normal room. Any recording made in that room will feature the echo, and the ear will not filter it to the same degree. Curiously, awareness of the original echo can be switched in and out at will. Try it.
I like the idea of instant record and playback. If I could PROVE what was actually said a couple of minutes previously, arguments in our house would be a lot shorter.