…such as those obtained from online video sites using a tool like this.

method #1: extract audio stream to AAC

Though according to this thread this may produce a corrupt file.

$ ffmpeg -i input.flv -vn -acodec copy output.aac

method #2: V0 MP3

Though of course this involves evil transcoding.

$ ffmpeg -i input.flv -vn output.wav
$ lame -V0 output.wav output.mp3

(source)