I don't know what apple is doing, but in theory, it should be possible to measure ohmic resistance of the coil, and infer temperature via PTC coefficient of coi

I don't know what apple is doing, but in theory, it should be possible to measure ohmic resistance of the coil, and infer temperature via PTC coefficient of coil material. The effect isn't very big at temperature differences you could tolerate in a loudspeaker coil though (~0,5%/K).