How to calculate the expected output current being drawn by a speaker?
In theory, your battery pack has 12.5 Wh capacity, so factor your average RMS power (W) and divide by efficiency to estimate hours.  As a Class D amp, it shall last longer with effic. So estimate 1W average and expect 12h.
Measure DC current with a 50mV shunt on ground side then test and verify performance. Speaker efficiency will affect results, so I might choose a 12” full spectrum speaker. This can be loud with 1W as I have done circa ‘70 with Philips speakers.