PAM8403 Class D Audio Amplifier
At only $1.31 per finished amplifier, I could not come close to building an amplifier for that price whether class D or not. They even have pads for a mute switch.
As you can see, they are very small; half the size of a 9v battery. The amps are stereo and rated for 3 watts per channel when powered by 5 volts.
I just received them so I have not wired one up for testing, but at that price, there is not much at risk.