As someone who builds speakers commercially, there aren't any devices that will actually monitor a speaker's temperature. I think what you mean is perhaps, wattage? The speaker's temperature arises from the coil, which is what moves to produce sound. In large speakers, I've seen manufacturers attach some sort of sensors pointing at the moving coil to produce an estimate, but for the speakers used in the Macbooks, they are so thin that this would be a near impossible feat. Maybe Apple uses an estimation from the DSP based on the watts supplied vs time. But, certainly, you cannot attach any sort of device to the coil without making it distort or non-functional.

I know that Apple like to supply the speakers with slightly more power (Watts) than they can deal with to get them loud, but I haven't seen nor heard of this temperature monitoring so far. I couldn't find anything related to it either. Please share citations, if you have.