Requires: SDL > 1.1.5, Avifile > 0.51
Here's the piece of the puzzle that'll actually let XMMS play .avi and .asf files. It's called AVI-XMMS, and it requires that you have avifilei0.52 or greater. It can be found at http://www.xmms.org/plugins_input.html#122 . Since we've already installed avifile (and the codecs) and SDL, we can start installing it as we would any other autoconf package.
Compilation took all of 30 seconds for me, and I don't have that terrifically fast a machine. Once installed, you're ready to go watch some AVI, ASF, or WMV movies!