Electrets produce quite a large output voltage compared to most types. If the Elephant is within a few hundred metres you may see voltages as high as 100mV from them so you don't need a lot of gain. I assume the battery is inside the microphone so you have a shield (ground) and 'live' wire from it. The amplifier probably isn't critical and an an op-amp should be more than sufficient. All it as to do is lift the amplitude enough that it overcomes the filter losses and still lets enough through to give a good indication on the DAC.
In general, tie the microphone ground to the amplifier ground and the amplifier ground to the filter and DAC. Keep then electrically joined but in a chain from mic to DAC. You only get ground loops where there are more than one ground connection and they are to points with different potentials on them. If you keep things in a chain, any potential difference is only referenced to a single stage.
Brian.