This guide will briefly outline how to play audio over mic using Linux. Everything used in this guide is included in just about every popular Linux distrobution or easily obtained from their software repositories.

There are two manditory dependencies. If you don't compile your own kernel, you shouldn't have to worry about these.
Kernel with ALSA support. (CONFIG_SOUND=y)
ALSA with loopback support. (CONFIG_SND_ALOOP=m)

Optional applications that make things easier:
PulseAudio (A featureful, general-purpose sound server)
pavucontrol (A GTK volume control for PulseAudio)
ffmpeg (Complete solution to record, convert and stream audio and video)
aplay (part of alsa-utils)