It depends on what distribution you're running and in what format you want it. It sounds like you're building Mplayer from CVS without having installed the libavcodec CVS.
You can grab the ffmpeg libavcodec source from CVS here (just hit enter when prompted for a password):
Code:
cvs -d:pserver:anonymous@mplayerhq.hu:/cvsroot/ffmpeg login
cvs -z3 -d:pserver:anonymous@mplayerhq.hu:/cvsroot/ffmpeg co -P ffmpeg/libavcodec
Håkan