The recording is "A Mighty Fortress" at Riverside Church, played by Virgil Fox on the Aeolean-Skinner Organ.
The sound amplification system we are trying out here illustrates how a high-efficiency loudspeaker system requires little electrical power to play at moderate volume levels in a medium-sized room.
Eight volts DC is supplied by a nearly discharged 9V battery. Supply current is monitored using a Fluke 87 rms reading DVM set on 39.99mA range.
Although it does not appear in this video, an analog sound level meter registers levels up to 80dB which is loud enough for demonstration purposes in this small room but louder than you would probably listen to normally.
The quiescent power supply current reads 21.44mA. Peaks of about 29mA occur during this video and the DVM does not over-range.
The difference between 21.44mA and 29mA is 7.59mA. At 8 Volts, this is 60.72mW. So, as an approximation to be sure, in my listening room, I can get 80dB peaks with 61 thousandths of a Watt of power supplied above the "quiet" current.
Qualitatively, the "large hall" ambiance is audible almost anywhere in the room. Loudspeakers like this can provide a convincing illusion of space much larger than your listening room and you don't need much electrical power to run them. You could even be "off grid". This video features a tiny class D stereo audio amplifier, Parts Express Part # 320-604.