This help article describes a browser limitation that cause the RadNotification to not play audio in mobile devices. You can also read how to avoid it.
You cannot play audio files in most mobile browsers from events like load or DomReady that do not interact with a user action.This is a browser limitation that can also be reproduced on a page that contains no Telerik UI controls (Example 1).
Example 1: Audio in browsers under mobile devices will not be played if triggered by the body's load event.
RadNotification exposes a public client-side method (userInitSound()) that lets you initializethe audio when there is a user interaction. You can add an event listener to the most appropriate element/control on your web page,usually the document itself, so that when the user makes an interaction with it, you can kick-start the audio (Example 2). You can also ensure that the sound is playable through the verifySound() method.
RadNotification doesn't attach an event handler to the document that kick-starts that audio when a user action is performedout of the box because such behavior contradicts with the one accepted in mobile devices.
Handling the corresponding event and calling the userInitSound() client-side method of the RadNotification would make sure that the sounds are available until the user navigates away from the currently loaded page or the notification control is disposed with AJAX.
Example 2: Enable the RadNotification audio in mobile devices by calling the userInitSound() method after a user action.