Options:
* Headset jack. You'd need an adaptor to handle the full headset port, and an app that will record over the headset.
* Bluetooth. Not sure if there is an app that will record over bluetooth, though, and I doubt the audio would be that good.
* Built-in microphones. Not the best option, but there are probably more apps that will work with the mics.
I don't think OTG supports audio input. That would be an interesting setup.