Set that Processing sketch aside, for now. Look at the example Processing sketches for receiving serial data. When you get one of them working, which really should be no problem (connecting to the correct serial port is the hardest part), you can modify that example to parse the data, and make decisions based on that data.
Being an engineer and a software developer, not an artist or musician, I can't see any relationship between periodic acceleration data and which mp3 file to play, but that is not the issue here.