When multiplexing audio do I need a pre-amplifier for each channel?
The range of microphones you are using appear to have a sensitivity of about -42 dB/Pa. This means they produce a nominal 8mV RMS for an RMS input sinewave pressure of 1 pascal. This pressure (1 pascal) is equivalent to 94 dB SPL (sound pressure level).

If you are measuring ambient sounds or even music, the peak/RMS level ratio (aka crest factor) can be as high as 20dB and this pushes the peak-to peak voltage output from the microphone up to 160mV from 8mV.

If you are measuring sound levels at up to 20dB higher than 94dB SPL, you might see an output from your microphone that is 1.6Vp-p.

You have a 2V range on your ADC so I'm questioning whether you need pre-amplifiers at all.