|Joe Bloggs said:
Does it specify the impedance used when measuring the voltage swing then? If not you can't make calculations like that--I mean if you take an 8 ohm headphone and still say the jack swings 305mV, you'd get 12mW instead of 3 or something like that...
It's true, the calculation assumes
that the unit can deliver 305 mV into a 32-ohm load. However, since many (most?) headphones for portable applications are in that impedance range, I think it's a reasonable assumption. But you're right, without a manufacturer-specified impedance rating it's not possible to definitively determine the power output.