With the help of from this website, I have been able to create an automator script in MacOS Big Sur (and others) where you can right click on a WAV file, and choose “Convert to MP3” or “Convert to 320k MP3” as per the name of your automation extension. This workflow sets it to a constant bitrate of 320kbps.
You can follow the steps at the above link, but I’ve just created a downloadable version.
Unzip it, and double click. Mac OS will then ask you if you want to install it.
It will then appear in the Extensions list.
This is the contents of the file:
This is the automater code required for making a quick action.
for f in "$@" do /usr/local/bin/lame -b 320 "$f" done
You will also need to install lame, via brew.
brew install lame
To locate the script once it’s installed, just right click on it and choose Show in Finder
For me, it showed up in ~Library/Services which you can then double click to be able to edit if you need to.