An obsolete Intuit / Roam Data credit card reader emerged from the heap:
“Turn up the volume” suggested where the power comes from:
They drive a LOUD, probably square-ish, audio signal through both “earphone”channels, rectify and regulate the output, and have plenty of power for the reader. The card data returns through the “mic” as another audio signal; I assume they choose an encoding well-suited for a dab of DSP decoding.
Nowadays, of course, 3.5 mm jacks are obsolete, audio data travels by Bluetooth, and you must keep a myriad batteries charged at all times.
The Smell of Molten Projects in the Morning 
“audio data travels by Bluetooth”
And money too…
https://krebsonsecurity.com/?s=bluetooth&x=11&y=9
Maybe the money just leaks out? [sigh]
“3.5 mm jacks are obsolete”
“If it’s good enough for Granddad It’s good enough for me.”
https://www.soundguys.com/ultimate-guide-to-bluetooth-headphones-20019/
Given my deflicted hearing, Bluetooth definitely isn’t the limiting factor.
Indeed, I can’t hear any difference between CD-quality and low-bit-rate audio …