Fun fact: if you go the other way around with
one of these TRRS breakouts, then get a
TRS extender cable, a
mono 3.5mm -> RCA adapter, an
RCA coupler, a couple of
3.5mm -> female RCA Y adapters, and a
mono cable, then you could use
the Dayton iMM-6 to get a calibrated measurement mic all-in for around $30, less than half the cost of a UMIK-1. And if you've already got any of those adapters or cables lying around or don't mind a used mic, you might even dip your total below $30.
Mic plugs into TRRS breakout. Red plug of splitter plugs into extender, with the green plug left dangling. Extender plugs into mono 3.5mm -> RCA adapter.
On your sound card, plug both 3.5mm RCA Y cables, one into the speaker out, and the other into line in. Use the RCA coupler to join white to white. Plug the mic cable into line in red, and plug the subwoofer cable into speaker out red.
Now you've got a calibrated measurement mic with a loopback timing interface and a cable for plugging a mono signal into your AVR or directly into your sub, whichever you choose.
Boom. REW measurements for cheapskates.