This is an old revision of the document!
MP3
Strip an MP3 of all metadata, not just ID tags:
avconv -i source.mp3 -write_xing 0 -id3v2_version 0 -map_metadata -1 -acodec copy no_metadata.mp3
See here for documentation of mapping tags using avconv.
Set metadata title:
avconv -i source.mp3 -metadata title="MP3 Title" -acodec copy title.mp3
Some more basics:
avconv -i source.mp3 -metadata title="MP3 Title" -metadata album="Knight Rider" -metadata track=1 -acodec copy metadata.mp3