I wanted to listen to a recent gubernatorial debate as a podcast, but I could only find videos of the debate online. A university was offering up a download link for the flv flash video on it's web site, so I downloaded that and used the following command to convert it to mp3. Since the video already contained mp3, using "-acodec copy" should have worked, but I was getting an error message likely related to a bad MP3 frame, so I just re-encoded to correct the errors in the file:
ffmpeg -i debate.flv -acodec libmp3lame GovernerDebate.mp3
Additionally, the following command would make an mp4/aac video file for playback on most video-supporting media players.
ffmpeg -i debate.flv -acodec libfaac GovernerDebate.mp4