Would it be possible to have an option to disable all sound on a video completely?

This feature doesn't exist.

This project is open source, so you could easily edit the C# code to add volume as a new initialisation parameter.

You could also easily use JavaScript to set the value of the MediaElement's Volume property to zero -- it would be one line of code if you use my SilverlightMedia JavaScript library.

Thanks,  i'll first take a look at the source code.