Mobile microphone input data throughput via audio jack?
There are two factors in determining the maximum theoretical information capacity of any communications channel, bandwidth and signal-to-noise ratio. The Shannon-Hartley Theorem places the upper bound of bit rate for a noisy channel to be:

$$ Bit Rate = B*log_2( 1 + SNR ) $$

If we assume that you have a perfect 20kHz bandwidth in the audio in channel for the smartphone, and 12 bits of resolution, the theoretical maximum will be around ~500Kbps.

It should be noted that achieving a value close this will require quite complex computations in the demodulation and that may very well prove to be the bottleneck of your system.