[morzh]

Well, if your manual says what load your audio output should accept, then you connect that load equivalent (say 50 Ohm if this is what it is - I do not know, look in the manual), turn the output to max, connect voltmeter and measure RMS value (which is the default for any today's voltmeter) and then go U-square over R.
Another question, why do you need to know power when knowing Voltage is enough.