Are you serious? Yes, I'm sure that a little heat would come directly from the voice coil, but that would just be the 'wasted' energy as far as the speaker were concerned. Most of the electrical energy supplied to the speaker would presumably be converted to sound energy, and that would have to 'go somewhere', even if no-one were in the box to hear it Is it really materially different from a lamp in a lightproof box - again, there would be no-one in there to see the light, but it would still heat up the box.I have a feeling that the heat would be coming from the voice coil. After all - if it's in a soundproof box there'll be nobody to hear it, so it won't produce any sound.
Kind Regards, John