Handy website which converts audio to other audio, video to audio, other formats: cloudconvert.com
https://cloudconvert.com/It is free to use, at first. If you do a LOT of converting (in my case, from mpg and mp4 and mkv, etc to mp3), they ask you to set up a free account. If you keep using it, they ask you to pay. You get 500 minutes of conversion time for $9. Not bad, to me, if you need to convert a lot.
= new reply since forum marked as read
The only problem with ffmpeg is that it is so powerful there are 1001 different options and browsing through the documentation can seem like getting a PhD in audio/video engineering. And a lot of people are turned off by a command-line interface 
But ... I'd be willing to wager that all of the audio/video processing on cloudconvert.com is running ffmpeg to do the actual work.
So ... if audio/video processing is something you do routinely, take the time to get ffmpeg and run it locally on your computer. It'll wind up being a lot faster than waiting for the uploads/downloads to any site in the cloud.
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