Once you have YouTubeToMp3 Youtube plugin installed you will see 'Convert to MP3' button appears. This button will ease the job on converting the video into MP3 directly from Youtube site.
But one thing for you to know, firstly before having this plugin to work exactly as it's advertised you need to have installed the original YouTubeToMp3.
Once you clicking the button: Convert to Mp3, you will see the YouTubeToMp3 launch and automatically start the downloading process. Is not that great, especially to those are notorious like converting Youtube video to MP3 file activity.
But this one is Cydia tweak, surely your iOS device must already been jailbroken before isntalling, follow the guide to untethered jailbreak using Corona for iPhone 4 or untethered jailbreak using Absinthe for iPhone 4s.
YouTubeToMp3 can be obtained for free in Cydia under BigBoss repository, then have fun guys!, Thanks for reading.