Admittedly my AC theory is a bit rusty, as am I,
but since a voice coil has an inductive component
to it shouldn't the formula incorporate the J factor,
(R+JX)? Or is this just a theoretical example of perhaps
a DC current being passed through the voice coil?

Mr Allison is quick to call me and others autistic idiots. Well, this
autistic idiot will be the first to give the correct answer.

The reactive component of the voice-coil impedance -- or any impedance --
doesn't dissipate energy. It's as simple as that.