This error usually happens when there are no MOH sound files uploaded or when default 'silence' file is not deleted.

• Login to the system administration
• Navigate to 'System: Music On Hold'
• Delete  'Default sound' file if one exists
• Upload new MOH file if there are no files uploaded yet and assign the file to 'default' MOH class
• Restart the system by clicking on the 'Restart' button

Now each call placed on hold will play all of the sound files located under the 'default' MOH class