ffmpeg がインストールされていることを確認

Debian Lenny ffmpeg 
paraches@debian:~/myTest/ffmpeg/ffmpeg$ which ffmpeg
/usr/local/bin/ffmpeg
paraches@debian:~/myTest/ffmpeg/ffmpeg$ ffmpeg
FFmpeg version SVN-r23512, Copyright (c) 2000-2010 the FFmpeg developers
  built on Jun  7 2010 17:24:52 with gcc 4.3.2
  configuration: --enable-version3 --enable-libmp3lame --enable-libtheora --enable-libx264 --enable-libgsm --enable-postproc --enable-libxvid --enable-libfaac --enable-pthreads --enable-libvorbis --enable-libfaad --enable-gpl --enable-x11grab --enable-nonfree
  libavutil     50.18. 0 / 50.18. 0
  libavcodec    52.74. 1 / 52.74. 1
  libavformat   52.68. 0 / 52.68. 0
  libavdevice   52. 2. 0 / 52. 2. 0
  libavfilter    1.20. 0 /  1.20. 0
  libswscale     0.11. 0 /  0.11. 0
  libpostproc   51. 2. 0 / 51. 2. 0
Hyper fast Audio and Video encoder
usage: ffmpeg [options] [[infile options] -i infile]... {[outfile options] outfile}...

Use -h to get full help or, even better, run 'man ffmpeg'
paraches@debian:~/myTest/ffmpeg/ffmpeg$

ちゃんとインストールできている！