This will become rather lengthy and complicated – basically none of those numbers have any relevance.
First of all, the amp’s output power has little relation to its technology power consumption.
Next, the rated maximum power has little relation to the average power used (which will, however, depend a lot on the volume you’re running your box at).
Then, a LiIon battery’s capacity is rated at (usually) 3.4 Volts – even if that specific power bank provides a 12 V output. Unless you’re using e.g. a power tool (or ebike) battery rated at (usually) 14.4 Volts.
In order to give 100 Watts output at 12 Volts, a typical class B amplifier will pull around 200 Watts from the supply – about 16 amps or so. The battery must be able to support that. A 12V/12 Ah SLA will be hard pressed, a 12(14.4)V/6Ah LiIon will find that easier, but will only keep up with it (at full power) for about 20 minutes….
If you want high volume and a reasonable runtime, you’ll have to go for efficiency on every corner:
– high efficency drivers
– a high efficiency speaker design
– class D amps
– bigger batteries