Is the Nintendo 3DS audio jack OMTP (LRMG) or CTIA (LRGM)?
I'm gonna have my dad solder a custom cable which connects the left audio tip of one TRS plug to the mic ring of another TRRS plug.Let me know how it goes. I tried doing the same thing; it didn't work for me.
I've made a crossover audio cable, but 3DS doesn't "listen" to it. The trick was a 1k resistor between MIC and GND pins.
-LEFT and RIGHT from PC goes to MIC of 3DS (with 1K resistor to ground)
-LEFT and RIGHT from 3DS goes to MIC of PC
Update:
I've made my own adapter.
- 3DS uses LRGM not LRMG
- You DO need the resistor between mic and ground. (I think that's how the 3DS detects that there is an external microphone plugged in.)
- Make sure there's nothing plugged in to the microphone jack when you plug the adapter in to the 3DS, otherwise it might not detect the microphone.


I just came up with this. It would allow mono headphone sound and two-way communication with maybe an online modem program or something.
Strange and impractical, though.
EDIT: The modem could also be a cell phone with a custom app, or maybe a web page with HTML5 or something like that.