Step 1: Free download FLV to MP3 Converter and install it.
The interface is like the following:

Condition1: you already have flv video files on your computer.
1. Click file button, choose the flv video you want to convert
2. In output format, Choose MP3 (Audio only).

3. Click start convert FLV to MP3.
If you happenly want to convert FLV Video to MP3, just have a try this excellent FLV to MP3 Converter.
Note: Converting FLV to MP3 is just part function of Emicsoft FLV Converter which is distributed by Emicsoft Studio.
For More Information about how to download YouTube files, just See this Guide: How to download youtube videos
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