Install USB sound card driver

install MP3 software:

opkg update
opkg install madplay
cd /mnt/sda1
wget --no-check-certificate  https://www.dropbox.com/s/zzowqtew6cmspsf/track39.mp3



ATmega32u4 Code:

#include <Process.h>
Process p;
void setup() {
  Bridge.begin();
  Serial.begin(9600);	// Initialize the Serial
  while (!Serial); // Wait until a Serial Monitor is connected.
}
void loop() {
  if (Serial.available() > 0) {
    int inByte = Serial.read();
    //Serial.write(inByte);
    switch (inByte) {
      case 'p':
        Serial.println("Play:");
        p.runShellCommandAsynchronously("/usr/bin/madplay /mnt/sda1/track39.mp3");
        break;
      case 's':
        Serial.println("Stop:");
        p.runShellCommandAsynchronously("/usr/bin/killall -9 madplay");
        break;
      case 'r':
        Serial.println("Repeat Play:");
        p.runShellCommandAsynchronously("/usr/bin/madplay /mnt/sda1/track39.mp3 -r");
        break;
    }
  }
}

p: Play

s: Stop

r: Repeat Play