[ubuntu] converting video error

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http://ubuntuforums.org – hi all i have question how to convert and extract audio from video file, i have try avconv but it says the codec was unknown i also write -acodec mp3 libx264 h264 and other codec but but the avconv always says unknown codec and so the winff, winff however can convert to wma or audio file but its corrupted, am i should download the codec? (Hardware)