between 0.5 & 1 watt of audio output? I think not - typical MP3/multimedia players have batteries which only store about 4-6 watt-hours of energy. If this thing will work for 5 hours on speakers playing video then that's only about 1 watt to run everything. Screen, decoding, sound - the lot.
In fact you can get a clue by how long it will run on audio only with speakers. It seems that you lose 27 hours of endurance. Assuming a 5watt-hour battery (probably an over-estimate given the size of the device), then on earphones it's using about 120mw, on speakers it's 300mw. Assuming that difference ot 280mw is mostly down to the speakers, and allowing for the inherent inefficiencies of audio amps, then you might be getting 150mw. Certainly nothing like 1W.
Perceived audio volume is logarithmic in nature. A doubling of power output (about 3dB) is only just about perceptable. Guessing power outputs from perceived volumes is no way to estimate the power output.