if you need to extract mp3 from Flv file(like youtube videos) this tutorial is for you
first of all download media player classic (MPC) from link below(Freeware):
Download Media Player Classic Home Cinema, Free Video Player Download
after that do the following instruction:
1-open MPC and go to File>>Utils>>DSM Converter
2-Drag your flv file to "DSM Converter" windows
3-Right click on "Audio" title and select "Demux"
4-In the "Save as" dialogue box rename the file extension to mp3(you can save it with aac extension if you want) and save the audio file anywhere you want.
I also extract the audio from wmw and mpeg