The ShowSound property does not resolve the server root symbol (~) so the request points to an incorrect URL, something like http://myHost/mySite/~/facebook.wav
Thus, you should
- either use a client-side resolvable URL like ShowSound="../facebook.wav"
- OR resolve the property on the server, e.g.: RadNotification1.ShowSound = ResolveUrl("~/facebook.wav"); for examplem in the Page_Load event.
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