This is what I had to do to get the right encoders to transcode.http://plugcomputer.org/plugforum/index.php?topic=721.0
the necessary encoders apparently are not included in the standard packages in the repository due to licensing issues, and the distribution for the ARM platform doesn't yet include the unstripped codec library.
Thanks for your suggestion. I download the ffmeg source and simply run the ./configure --enable-share --prefix=/usr and follow by the make ; make install. Will this compile all encoders or I should put more options onto the ./configure?