Thanks misterT for the great information. I studied the links and still have some questions on that.
Though an AVR micro controller can generate PWM signal and drive speaker directly, I may need a speaker amplifier to drive a more powerful speaker (lets say 2W).
Is there a speaker amplifier chipset supports PWM signal as input? Because when I searched the speaker amplifier chipset I noticed they generally support digital or analog input. I don't know if I can treat the PWM signal as digital signals here?
In the case of no PWM input speaker amplifier, what should I do to use convert PWM signal to digital/analog signals required by the speaker amplifiers?