I will assist some more with a formula.

P=I*E

P = Power(watts)

I = Current(amps)

E = Voltage(volts)

Now you need to figure out basic algebra and how to use a calculator

Actually this gives you Volt Amps.

Now the Electrical Engineer side of me wants to toss in that this actually only works with DC to get to Watts. When you look at Watts in AC, It's Volts times Amps times Power Factor. Power Factor is the Cosine of the Angle between Volts and Current. Capacitive circuits will lead in Power Factor, (Current is ahead of Voltage) Inductive Lags in Power Factor.(Voltage ahead of Current) This is why Generators have 2 power ratings, KVA and Watts. Don't exceed either to live long and prosper.

Speakers are Reactive, Meaning they can have Inductive or Capacitive traits. Just measuring the Voltage, and using that across the specified Impedance may be close, but there is much more happening in that circuit.

AC power is a different animal than DC, Just keep that in mind as terms are tossed freely around. Current times Volts will get close, but could also kill you for not knowing there is a difference.

Bob T