Hardware serial port supports 115200, 230400 data rates.
SPI can send data at 8 MHz rate, but that requires 2 pins, clock & data.
The onboard ADC takes 110uS/conversion, so that's really the limiting factor.
You can use an external ADC with SPI interface to sample the Mic a lot faster,
See Figure 6.1
You need to set SPI.divisor to 8 for 2 MHz clock. Then:
PORTB = PORTB & 11111011; // D10 low for chip select
lowByte = SPI.transfer(0);
highByte = SPI.transfer(0);
PORTB = PORTB | 0b00000100; // D10 high
// combine into an int, something like this - always takes a couple tries to
// get casting & stuff done right
combinedData = ( (highByte <<8) | lowByte) >>4;
Whole thing takes about 10-12uS or so.