As of 2014, MP3 Rocket is a legal YouTube-to-mp3 conversion software, according to CNET. However, how the files are used after conversion determines legality. Using the software to download and distribute copyrighted material is illegal.
MP3 Rocket is not a licensed entity. It's an open source, P2P-file-sharing software that's similar to Limewire. According to CNET, the Creative Commons filter on the software allows users to filter out potentially copyrighted material. Any content that is downloaded and distributed illegally is subjected to infringement liability. Copyrighted music or video that has been obtained without payment and duplicated by the consumer is considered illegal.