Shouldn't this be used to start Windows Media Player?
According to CLSID, it should.
So, why would someone obfuscate the call (part of the call is in variable) if he does not have something to hide?
There was probably some downloaders using the same method, and as we have nothing else that can be used as signature, only pointer is that a part of call to WMP is made in a variable (variable mp in this case).
That would be my guess.