Assuming you're running windows go to view tools, folder options, click the filetypes tab, click on any of the items the massive list of file extensions and type flv. It'll jump to it. From there you can change the default flv player, or click NEW to add one.
Howto Capture the Audio or Music From Youtube Videos
Use the demo of goldwave or the freeware audacity with lame mp3 codec for converting to mp3 format if you want to record songs over 60 seconds.
How to Convert the FLV to Another Video Format
Maybe you want to convert the flv to avi or wmv which you can edit in windows movie maker, virtualdub, adobe premier, or mac's final cut pro. Just watch this video tutorial howto lecture.
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