Here is a nice and entertaining project created by http://www.ermicro.com . The author has a very good programming, teaching, drawing and artistic skills. The tutorial is well planned and executed. I really liked the RGB LED and Sound show given at the end. I will try it and tell how it works. Till then you can read the rest of the article here.
The article also explain various PWM techniques like Fast PWM, Phase correct pwm, phase and frequency correct pwm. You will also learn tone generation using PWM technique.
For more detail: AVR RGB LED and Sound Show using ATmega168 microcontroller