Boost converter noise
There are two potential problems with the booster you've chosen. Firstly, it's specified as having 30 mV ripple and if your Mic circuit is producing output signals close to the supply rails then you'll get this ripple superimposed. Secondly your amp has a power supply rejection spec that may be poor and this will produce noise on the feed to  your ADC. Thirdly, the switcher is a type that works best under medium load conditions and if your amp is only taking 10mA or less the switching frequency will be quite low and possibly ripple will be higher. Try loading the 3v3 with 100 ohms and see if this improves things.

Fourthly, your electret Mic power feed should have two resistors from the power rail and a 10uF cap down to ground to further reduce supply ripple superimposed on the Mic signal.