Hi!
As mentioned on the product page, by default the output power is 0.45W. In general you shouldn’t run speakers at their rated maximum power, as they won’t sound very good that way. For more volume it’d be better to use a larger speaker.
If you do have a larger speaker (or don’t mind some distortion) you can increase the gain by 3dB by shorting the open solder bridge close to the amp chip, or by 6dB by soldering a 100k ohm 0603 resistor there. You could also try increasing gain in the mixer software.