Arduino Micro + Max4466 - Noise on A0
The OP itself solve it:


  Ok - so I suspected that it was the Arduino at fault.. Turns out it's
  a cheap Chinese clone. After purchasing an official Micro, and wiring
  up the same component, I'm getting a nice clean signal (bar a small
  amount of acceptable noise), which is reflecting input tones
  accurately. Thanks again for all the input!