With a 5V power supply the theoretical maximum power is 0.4W into 8-Ohms or 0.8W into 4-Ohms. In the real world there will be some voltage-loss through the amplifier so you'll get less. (5V peak-to-peak is 1.75VRMS.)

Perhaps by using a speaker with a higher impedance?

With higher impedance, you'll get less current (

Ohm's Law) and therefore less power. Resistance (and impedance) is the

*"resistance to current flow".*If you double the voltage, you also double the current so doubling the voltage gives you 4 times the power*, whereas halving the impedance just doubles the current giving you twice the power.*

I believe the minimum voltage gain of the LM386 is 20, and since you can get 5V peak-to-peak out of the Arduino you don't need much (if any)

*voltage*** gain and you can easily drive it into distortion. (And, you

*might* get better

*quality* if you add an analog volume pot so you can run the digital volume "hotter" and reduce the analog volume.)

* Within the limits of the chip.

** You do need

*power* gain.... More

*current* than the Arduino can directly supply.