When creating a Music Source using a mp3 file, the file must be stored on a sharing service that allows unrestricted downloads. The download URL is what is needed in the URL field, not a sharing link.
To use google drive, the file must be shared with anyone with the link. Then, after downloading, take the URL of the download from the download page of the browser, not the shared link, and place it in the URL field of the Music Source.
The Google Drive Direct Link Generator is needed to convert the shared link to a downloadable one.
If, on test of download after converting the URL, you get this message, it's not going to work the file is too large:
Using the sharing link of a dropbox file, change the 0 to 1 at the end of the URL. (?dl=1 instead of ?dl=0 at the end of the URL.
Retrieve the file download URL from your downloaded files list in a browser.