Tank you for your feedback.
In order to set the property ToolTip of the Button in the RadMediaPlayer you have to make a custom theme of the control in Blend.
Please find attached a project with a custom theme of the RadMediaPlayer and setting the ToolTip property of the following buttons (FullScreen , PlayList).
If you have further questions please do not hesitate to ask us.
I hope this will help you.
the Telerik team
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