If you Google for a tool to convert Youtube flv videos to mp3, without a doubt you would find more than 100 tools, but I am quite sure none of it can match the feature rich interface provided Dirpy.
Dirpy is a free online Youtube video to mp3 converter with lot of power packed features.
When you enter a Youtube video link in Dirpy, it processes the flv link and gives you many different options to customize the mp3 file before you can actually start downloading the audio file.
So one can select start and end time from video to convert to mp3 - something like a mp3 cutter or clipper tool or can be used to create ringtones, it also allows to select bit rate of 64 kbps or 68 kbps or 112 kbps or 128 kbps or 256kbps for mp3 file, more you may even set the ID3 tags like title, artist, album, year, comment, etc for the mp3 file before you actually start download.
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