Silly question but did you unplug the midi shield and plug in an mp3 player or ipod or cell phone or other device to test your amplifier isolated from the Midi shield ? (You can use a Line out from your PC to the Line In of the amp)
..is that 12V backed by a charger?I've notised that many 'intelligent' chargers use PWM at 2.5 kHz to charge batteries. That freq often tend to end up as an irritating sound. (It did for me)
Simply connect a speaker/stereo/pair of headphones to the 1/8" stereo jack on the shied and pass the proper serial commands to the IC and you'll be playing music in no time!
plug stereo line level into the 3.5mm stereo headphone jack
Above the pinouts there are three solder jumpers (Analog / AD1 / AD2) and then on the left, a 3-pin breakout called Pot. Vol - these are used in analog mode, the solder jumpers are closed to tell the chip we'll be using a potentiometer to set the volume. The Pot Vol connection is how we wire up the 1K potentiometer. This is covered in more detail in the Analog Wiring section.