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. Files must be recorded in 2 channels. If the file uploads but there is no audio on hold, the cause is usually that it's recorded in. mono/1 channel.

Google Drive

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 or https://drive.google.com/file/d/DRIVE_FILE_ID/view?usp=sharing

The drive file id will be this-

Characters are blurred intentionally.

Copy just the file id from the link and paste it into the link below replacing DRIVE_FILE_ID

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.

Dropbox

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.

Example: https://www.dropbox.com/s/i6ahce3e1q411ww/CEO%20Welcome%20Audio.mp3?dl=1)