Just as an addendum. The efficiency of a loudspeaker (measured in decibels-dB) will have quite a marked effect on volume. Let's assume a loudspeaker with arated efficiency of 88dB. An increase in efficiency of just 3dB to 91dB, and given an amplifier of the same power, will increase apparent volume x 1.23.

I don't know if the tiny speakers used in our models are thus rated but it would be a straightforward swap which needn't involve anything more complex than soldering two wires, and voila, a much louder tank.