Have you tried putting your control inside a border? Just set the border's corner radius to 150 and you have a perfectly round control. Here's an example with a button.
<Border BorderBrush="Black" BorderThickness="1" HorizontalAlignment="Center" Height="200" Margin="0,0,0,0" VerticalAlignment="center" Width="200" CornerRadius="150">
<Button x:Name="btnPlayback" Content="Play" HorizontalAlignment="Center" Height="200" Margin="0,0,0,0" VerticalAlignment="center" Width=" 200" BorderThickness="0" Click="btnPlayback_Click_1"/>
</Border>
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