Convert YouTube Videos with Maximum Quality Retention

The easiest way to download videos from YouTube is perhaps by using the VideoDownloaderHelper extension. It can not only download videos from YouTube, but from almost any website that features embedded video.

You need ffmpeg to convert videos.

1. Videos to Mp3:

ffmpeg -i video.flv -vn -acodec copy file.mp3

Note that some videos feature the newer aac codec, so the above won't work. Use
ffmpeg -i video.flv -acodec mp3lame -ab 128k file.mp3

2. Video to iPhone:

ffmpeg -i vcodec copy -acodec copy file.mp4

3. Video to Mobile:

ffmpeg -i video.flv -vcodec h263 -acodec copy -b 200k -s qcif file.3gp

There are what I use, and might not work for an older phone.