Alex, that's curious unless maybe it's just a matter of semantics, i.e., when Ohm's Law is used in its AC form some think it shouldn't be called Ohm's Law. However, it is of universal application and a quick googling turns up numerous examples illustrating application to AC, for example here
. Of course, this is using the Z factor rather than R to take into account varying impedance with frequency.
Certainly the calculations for impedance are more complex than for pure resistance, but the point is that once done the result is a specific current: same speaker, same frequency, same input voltage results in a fixed current which can't be changed by changing the amplifier.