aTube Catcher lets you download videos as MP3 files.

If you want to convert music videos into high-quality MP3 files, aTube Catcher is a great app to use. You can use the app interface to browse through the internet and convert any videos you find into audio files. Although you'll probably use this for music more than anything, you aren't strictly limited to downloading songs. You can also convert and download speeches, lectures, narrations, or any other content in video form that you would like to listen to in your audio playing app. The app is also free, which makes it a great alternative to other apps that charge you for this service.