Mencoder Usage

Listing available codecs:
mencoder -ovc help
mencoder -oac help

Basic encode syntax:
mencoder movie.wmv -o movie.avi -ovc lavc -oac lavc (mpeg4 default)
# -o = output file
# -ovc = output video codec
# -oac = output audio codec

One-pass FFmpeg (lavc) encoding:
mencoder file.avi -ovc lavc -oac lavc --ffourcc DX50 -o output.avi
mencoder file.avi -ovc lavc -oac lavc -lavaopts acodec=mp3 -o movie.avi

http://gentoo-wiki.com/HOWTO_Mencoder_Introduction_Guide