MP3 appears in iPhone music library but not seen when connected to MacBook

I have an MP3 of an audio book that I converted myself from cassette tape and loaded into my Music Library on my iPhone. It appears in the library, and is played in Music.

When I connect my iPhone to my Macbook and open my music library on the iPhone, it's not there.

I need to save it to my MacBook, but I can't if Music doesn't "see" it.

How do I resolve this?