The Auxiliary input is designed for an amplified signal such as what is output from a smartphone headphone output. In order to use a microphone with the Aux input, it would need to be used with a microphone preamplifier before the signal gets to the Livemix Aux in.
But as luck would have it, we make just such a device. The LM-MICPRE is a mic preamp that allows you to use a standard dynamic mic as the intercom mic. Check it out here.