Since your Speakers have a Specific Impedance (Resistance) (In Theory Typically 2, 4, 8 or 16 Ohms), You don't need to measure the current.
Just Make a peak detector citcuit with an Analogue voltmeter on the output.
Knowing the Resistance (Impedance)
The wattage is Volts Squared/Resistance
Example 10 volts at 8 ohms would be:
10 times 10
-------------- = 12.5 watts
8
Than RE-Calibrate the scale of the Voltmeter to read directly in watts.
This is Not a Precise wattmeter, But for Simplicitity, typically Close Enough.