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. Copy the link.
The link will look like this- https://drive.google.com/open?id=DRIVE_FILE_ID/view?usp=sharing
Replace open?id= with uc?export=download&id= and delete /view?usp=sharing
The new link will look like this- https://drive.google.com/uc?export=download&id=DRIVE_FILE_ID
Note-the File ID is much longer- DRIVE_FILE_ID is used as a placeholder
To test- copy/paste the new URL in a browser window. If the file downloads immediately, it's correct.
NOTE- As of Summer 2019, Dropbox Professional is required for the necessary sharing settings. No longer available with free accounts.
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.