TubeMate YouTube Downloader - This post explains about 'Convert' configuration. First, you need to enable MP3 Video Converter (requires an additional app installed from play store) for download audio (AAC/MP3) and video
How to convert MP3 to 320kbps
Free YouTube to MP3 Tool | YouTube MP3 Conversion Tool
Free YouTube to MP3 Tool | YouTube MP3 Conversion Tool
How do YouTube-to-MP3 sites work? How would one go about building such a site? What libraries/technologies are used? - Quora