Your XML reads:
Visibility="{Binding ElementName=MixControl, Path=Items/Count, Converter={StaticResource GreaterIntegerConverter}, ConverterParameter=0}">
Yet, the function that you want to call is:
NeonMika.EightTracksPlayer.GreaterIntegerToVisibilityConverter.Convert
So it seems to me your XML should read something like:
Visibility="{Binding ElementName=MixControl, Path=Items/Count, Converter={StaticResource NeonMika.EightTracksPlayer.GreaterIntegerToVisibilityConverter.Convert}, ConverterParameter=0}">
But even that seems a bit dubious, since you've declared GreaterIntegerToVisibilityConverter as a class and Convert as a non-static member of it.
So unless you are instantiating a GreaterIntegerToVisibilityConverter somewhere prior to that line of XML the Covert member function isn't accessible from anywhere.
And if you are instantiating one somewhere, it's not at all clear how the thing you instantiated elsewhere is actually bound to something that is known by the XML interpreter at the point it tries to evaluate Visibility.
Set a break point on your convert function and run it in the debugger -- I'm pretty sure it never gets called.
UPDATE:
Assuming you have something like this earlier in your XAML:
<local:NeonMika.EightTracksPlayer.GreaterIntegerToVisibilityConverter x:Key="GreaterIntegerConverter"/>
then you should be binding to the correct converter function -- if that's the case, the problem lies either with the element or the path.
Assuming the element is spelled right, then maybe there is an issue with the Path.
Try changing the path to just "Items" and have your converter extract the count from the collection.