You can also have youtube-dl convert to an mp3 automatically after the download by using the -x option like so:

youtube-dl -x --audio-format mp3 https://...

You'll still need ffmpeg installed.

As far as a tool that supports dtube I know of none, but the best bet would be if someone added that support youtube-dl. It already supports many other sites. https://github.com/rg3/youtube-dl/blob/master/docs/supportedsites.md