Mike Melanson and Mario "HCl" Brito have compiled some very technical research into the codecs and file formats used in the later Wing Commander games. The Multimedia Research Institute Site has added a number of documents concerning these details. Information on Wing Commander 3's MVE multimedia file format can be found here. The famous Xan codec is covered here, though currently it covers the Wing Commander 3 variant. And TRE file research for WC3, WC4 and Prophecy can be found here. And more is on the way.
I am a multimedia hobbyist, historian, researcher, and fanatic. I maintain a little website called the Multimedia Research Institute (MRI). The MRI carries as many useful, specific, technical details about computer multimedia formats as I can possibly track down and publish.
Edit: Got another piece of info here.
I neglected to mention another useful piece of data on the site-- The following document has a collection of simple audio coding algorithms, including the Xan DPCM algorithm used in Xan AVI files: "Simple Time Domain Audio Coding"
and Xan DPCM is here.