Every earphone manufactured nowadays (the type that inserts into the ear canal), seems to have a mic built into the cord.
I'm looking for a set for music only, not phone use. I figure, the in-line mic must introduce distortion. But I'm not sure, maybe it's filtered or bypassed below perception level. What do they look like, internally?
The sleeve (closest to the connecting wire) is common ("ground").
The tip, and the ring closes to the tip, are the two audio-out channels (earphones). The connected device drives audio to them, against ground.
The second ring (closest to the sleeve) serves the microphone, and (in a lot of cases) a single push-button which acts as a control. Typically, the ring goes to an electret microphone (whose second terminal is connected to the sleeve/common/ground). If there's a pushbutton, it's connected across the microphone (between ring-2 and ground) and shorts the two together when pressed. The connected device will typically provide a bias voltage to this connector (e.g. +5 volts DC, through a series resistor), and also capacitor-couples the connector to a microphone-preamp circuit. The device often monitors the DC voltage on this connector to detect cases when the pushbutton is pressed.
So, the microphone circuit in the 'buds has very little impact on the earphone audio quality, because all they share is the common/ground wiring. The "hot" audio signals don't reach the microphone at all.