First log into https://myvoiceportal.com. Log in with your credentials, you must have elevated credentials to make this change. Contact support if you need to have your credentials elevated.
Now click where it says "Manage Organization" at the top right
Now click on the icon at the top that says "Music On Hold"
Now click on "Add Music" at the top right
Now browse to the music on hold file you wish to upload. It needs to be either a .WAV or .MP3 file and not be copyrighted material. Type a name for the hold music in the "Song Name" field. Now press "Upload"
That is all there is to it. Once this sound file is uploaded it will be the new on-Hold and Park music for your domain!