In wiki it says to make the 3.4 kernel to run with Android 5.1. But 3.4 is not the standard kernel to use with Android 5.1, standard is 3.10. On github you have kernel 4.x available for Linux use. If Android support is turned on in that kernel does it work with the Android 5.1 user space? Kernel 3.4...

The schematic for both NanoPi2 boards mentions I2S, but I don't see how it is routed to a connector where I can get to it. Is I2S accessible on either NanoPI2 board?

Have not tried this on NanoPi, but USB Audio support is a kernel option. Make menuconfig on the your kernel and turn it on.