I've just uploaded a 1Mb zip file of dlls for the windows version of plague. Unzip them into the same directory as the plague.exe. This fixes the missing "mixer" error message.
This seems to be caused by developing on Win7 - weird. It's kind of ugly having all the dlls there, so I will probably compile on winxp from now on (doesnt seem to be a quicker answer for sorting this out, but I'm happy to be proven incorrect).