How to download youtube video, convert to mp3

-You can use one of the many programs. One is Jdownloader.

-Open Jdownloader.

-Find the video on youtube. Copy its address: Right click on "", and select "copy"

-Open Jdownloader.
-Select "title of the song.tmp" in the Linkgrabber tab. 
 It is under mp3 section.

 If there are no links in Jdownloader, click "Add url(s)" at the bottom of Jdownloader window, on Linkgrabber tab.

-Rigt click on mp3, select "continue with selected link(s)"

If you want to convert/extract mp3 from already downoladed youtube videos, you can use gulmencoder3, (version 3).

-Drag and drop file(s) on gulmencoder, 
-click on "+set\audio"
-click on "convert audio to another format using -format setting (ffmpeg)"

-(Optional change "-ab 160k " to "-ab 128k ", if source quality is not of high bitrate.)

You can also use gulmencoder3 to split/merge mp3 audio files.