VVMicVST is a VST plugin that implements an ambisonic decoder. It takes B-Format files as input and can produce output for stereo, 5.1 or many other speaker configurations. It supports up to 8 outputs.
VVMicVST is free, open source software. See the Source Forge project.
B-Format is a general purpose representation of the soundfield based on spherical harmonics. For 1st order, as used here, this consists of one omni and three figure eight pattern signals called W, X, Y and Z.
Ambisonics is a method of recording and reproducing surround sound that was originally developed in the 1970s, mostly by Michael Gerzon. For more information about ambisonics see ambisonia.com.