Step 3: Getting Control...
First, a little explanation of how the keypad is wired on the PCB. Take a look at the Connections photo. There are 4 unique connections between the rings and bulls-eyes. But one of the connections is ground based, so we don't have to attach a wire for that connection. The ground based connection allows control of the PLAY/PAUSE, VOL+ and VOL- control. This is simple, we just use three of the Arduino's IO pins to ground the bulls-eyes for these three signals. SInce the MP3 player is running at 3.3V and the Arduino is running at 5V, we simply make these 3 pins INPUTS until we need to ground one of them. Then we make them a LOW OUTPUT. No excessive voltage is ever applied to the MP3 signal lines.
The purple, orange and red dots are where you want to solder some wires (I used wire wrap wire.) I connected VOL+ (purple) to Arduino pin 10, VOL- (red) to Arduino pin 9 and PREV (orange) to Arduino pin 8.
To make connections purple and orange (PREVious) and red and orange (NEXT), you will need TWO tri-state buffers or TWO analog switches. Luckily I had some analog switches from a previous project. (There are probably more clever ways of making these connections but there are some subtle problems to overcome.)