Multiple-microphones and mic-switcher
The microphone signals are probably single-ended, and then you want pre-amplification at the microphone end of the cable. If you would transfer the low-level signal over the cable first and then amplify you would amplify any picked up noise as well. Your S/N ratio could be pretty bad. Balanced signals are far less susceptible to this, but usually only professional microphones use this. Most of the time they'll use an XLR connector.

For the switching you could build a summing amplifier like this  

  

and use a 74HC4066 analog switch for each \$R_{IN}\$.