Technology

Most likely the mic is simply powered by the voltage which also powers the headphones.

But there's also mics which don't need voltage to work at all (unlikely for headsets tho).

Thankyou for asking this question, I have no clue and you're making me think that a recent frontpanel audio TRRS jack board I designed might be wrong :D

There are two possible options I can see:

1. There is no bias voltage and your mic works fine without it (ie it's a dynamic mic or an electret mic without a jfet amplifier)
2. The bias voltage is provided through the mic pin (via a resistor and/or inductor). The mic then overlays AC onto this DC signal.

I cannot find any good references or info about mic bias and TRRS connectors :( Anyone else have any luck? Wikipedia says it's a standard referred to as "CTIA" or "AHJ" but those appear to be company names, not standard names.

My current headset uses a TRRS, but also provides an extension cable that splits into two 3.5mm TRS just like yours. I might probe it out and find out what it's doing (but that doesn't mean it's the right/universal solution).