FFmpeg can extract the audio from FLVs without re-encoding to preserve the quality.
If you don’t already know what type of audio is in the file, do :
$ ffmpeg -i inputfile.flv
Along with some other data, it will tell you about the audio:
Stream #0.1: Audio: mp3, 22050 Hz, mono, s16, 56 kb/s
As mp3 audio was used (which is usually the case in FLVs), perform the following command to extract it to output.mp3:
$ ffmpeg -i inputfile.flv -acodec copy output.mp3
Alternatively, for aac audio streams, you can use:
$ ffmpeg -i inputfile.flv -acodec copy output.**aac**