Since Q3 2011 the RadNotification offers the ability to play a sound when it is shown so that the user can notice it even if the page is not active. This is turned on by setting a value to the ShowSound property. You can pass an URL to your custom sound, which must be in wav format.
Just like with the icons, there is also a set of predefined sounds built-in the RadNotification that you can use directly:
Audio in mobile devices can only be played after a user action. This is a limitation of mobile browsers that also affects the RadNotification control. You can find more information and a possible solution in the Notification Audio Is Not Played In Mobile Devices help article.