Electronic – Pre-amplifying an electret mic already in use

You could do something like this if you're not afraid of op amps. The microphone gets its high impedance, and if it needs higher impedance, increase R1. I have the iPod a 330 Ohm load because I seem to remember reading somewhere that's a good starting point. The output impedance is 10k, and can be increased if necessary. This circuit has a gain of 2, so if you need less, decrease R6, and if you need less than 1, switch it to an inverting configuration. Trying to sum these inputs passively is going to require a matching network.

What are the steps required to join the microphone and audio lines from a typical cell phone headset into one line so that the audio can be recorded by a standard voice recorder using the microphone and/or line-in input?

In this context "audio line" means a signal level & output impedance intended to drive a typical pair of earphones. I'll assume mono rather than stereo.

I'd guess the processing steps may be not entirely unlike the following:

1. for mic-in (could be entirely passive?)

   1. Attenuate the headphone signal to mic levels
   2. mix audio signals
   3. buffer output

2. for line-in (choose either pro or consumer level)

   1. amplify the microphone signal to line-level.
   2. amplify or attenuate earphone signal to line-level
   3. mix audio signals
   4. buffer output

By buffer output I mean set the appropriate output impedance.

For stereo, you'd typically split the mic signal and mix equally into left and right channels.

For recording conversations you might want separate automatic gain control (AGC) for each side of the conversation. Since conversations are mostly half-duplex I guess you mostly only need one AGC element.