Your FFmpeg configure line is missing --enable-libmp3lame. This is required if you want to encode mp3, although the error message could be improved. I assume you followed the 10.04 version of the guide and skipped step 5 or simply omitted the instructions to add --enable-libmp3lame. This happens on occasion, and I will consider changing the guide because I feel that note about adding --enable-libmp3lame is often ignored or overlooked.
What you can do:
- Recompile ffmpeg and follow step 5,
- Or Pipe from ffmpeg to LAME,
- Or uninstall your compiled ffmpeg and install it from the repository instead
If you're impatient just install lame:
Then pipe the audio from ffmpeg to lame:
sudo apt-get install lame
Or install ffmpeg from the repository and enable the mp3 encoder. It's easy to do since you'll just need to install two packages and is described under Option B here:
ffmpeg -i input.mp4 -f wav - | lame - output.mp3
HOWTO: Easily enable MP3, MPEG4, AAC, and other restricted encoders in FFmpeg
Then you can use your same command from post #1950, but I recommend adding -aq 4 to improve the quality of the output because the default settings use an unusually low bitrate (64 kilobits/s [but maybe default is 128k now]) for most purposes.