What level and/or impedance matching (or pre-amp) circuits should be used to send a line-level audio signal into the mic input ring of an iPhone headset jack with minimal distortion and/or loss?
Line levels are 0.5-2.0V range. Mic inputs are expecting millivolts. You just need what is called an L-pad attenuator. Your drop should be 40-50 dB.
If you are using a consumer line level, you will need about 40 dB attenuation. (Professional audio line input would need about 50 dB.)
Attenuation in dB = 20 * log10((R1 + R2) / R2)
Try R1 = 10k and R2 = 100.
If you get accurate measurements of your line source and a know good mic signal for the iPhone, you will be able to calculate more accurate values. Or you can put a 500 ohm trimmer resistor in place of R2 to allow varying of the signal if the line out doesn't have adjustment.
I am assuming you mean the inputs through the Left/Right inputs, in the 30-pin connector. I've tried to find that info. At least you don't need an analog to digital converter.
If you mean to use the mic ring on the 4-way plug, there is sensing voltage to be aware of. It has to do with the typical volume/mute controls, for inline mics in headsets typically used with the iPhone. I learned this, due to needing to match a low-impedance guitar pickup, to that mic input. One circuit, for my use, is described at: http://groupdiy.com/index.php?topic=46767.0 and the diagram can be downloaded at: https://imageshack.com/f/ghiphonemicinputj