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.
Click this bar to view the full image.


PS:
I also extract the audio from wmw and mpeg