Since I'm no longer with the company, the email address is no longer valid.
Correct, if you no longer work for that company, that company has already deleted your work email address. This is done as soon as you have left the company so that you can no longer log in. You no longer have any rights there.
An deleted email address, it is no longer valid, it simply no longer exists and it no longer works.
You may not be able to change that email work address because it is no longer exists.
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I have a new email and want to authorize the music with that one in order to listen to it. How can this be done?
That is not possible, sorry.
All apps and music purchased with that work Apple ID, no longer exist.