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 the URL you get, when pasted into a browser, streams the music and does not request to be downloaded, the file has not been converted properly and will not play when added to OnSIP Admin Portal as your music on hold music source.
If, on test of download after converting the URL, you get this message, it's not going to work the file is too large:
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.