Got the same issue and question here. Looks like the Youtube mp3 converter was taken offline due to threat of lawsuit. I have spent the morning trying to find a free alternative that is clean (no ads, hidden costs, etc) like like the Youtube converter. Most everything I have looked at either has limitations on downloads, tons of ads, or junk that runs in the background trying to access contacts, locations, phone, etc. If someone knows of a good replacement, I'd be real interested.
Downie. It will give you the ability to download video from a lot of different sites, as well as extracting audio from YouTuibe videos etc to mp3. I have been using the software for several years and have had no problems. It works well and the developer is great with user response and updates.