The power amplifier on the Bela rev B cape can max do 1.1W per channel on an 8ohm load. The latest rev C cape has a new power amplifier that can get significantly louder than that. The part is this. This is an annotated version of the power specs from the datasheet (all powers are per-channel):
By default the amplifier is wired in class-AB (so it can deliver up to 3.14W on 4ohm at 10% THD), but by cutting a solder jumper on the top PCB, it can be set to class-D and reach up to 5W on 2 ohm at 10% THD. Once in class D mode, one has to be extra careful not to disconnect the speaker while the power amp is on, or it will get damaged.
In none of these cases (neither rev B nor rev C) you can bridge the two outputs, because the outputs are already fully differential and not ground-referenced. Ultimately if you need more power than these you'll need a power supply with a higher voltage, because the PAM8406 already squeezes as much as you possibly can out of a 5V supply ...
