Automator application that asks me for a YouTube URL before running the command
Through lots of trial and error I finally managed to actually download and extract the audio. However I'm getting an error message: &quot;ffprobe/avprobe and ffmpeg/avconv not found. Please install one.&quot; Thing is my ffmpeg is fully up to date. I even reinstalled it just in case. My &quot;youtube-dl&quot; is also up to date.

Addressing your comment...
The default PATH passed to a Run Shell Script action in Automator is:
/usr/bin:/bin:/usr/sbin:/sbin

Add a PATH statement at the top of the code in the Run Shell Script action,
To determine what the normal PATH is, in Terminal run echo $PATH and then for the Run Shell Script action use whatever it outputs, e.g.
PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

for f in &quot;$@&quot;; do
    /usr/local/bin/youtube-dl -x --audio-format wav -o &quot;$HOME/Downloads/%(title)s.%(ext)s&quot;  &quot;$f&quot;
done