Froggy14's code with corrections as to allow it to get compiled.

froggy14_MP3_PIR.ino

// libraries
#include <SPI.h>
#include <SdFat.h>
#include <SdFatUtil.h>
#include <SFEMP3Shield.h>

// initialize MP3 card
SdFat sd;
SFEMP3Shield MP3player;

// constant variables
int pirPin = 5; // PIR sensor input pin
int calibrationTime = 10;
unsigned long playTime = 15000; // how long a file will play in milliseconds (15000=15 seconds)
unsigned long pauseTime = 10000; // how long the pause will be after the sound ends (10000=10 seconds)
int readingInterval = 20; // how often to read the sensor

// changing variables
int rantrack = 0; // track number for randomizing
unsigned long currentMillis = 0; // time variable to track running time: current time
unsigned long startingMillis = 0; // time variable to track running time: starting time
byte result; // variable for mp3 player shield, can be used to debug

// setup
void setup() {
  pinMode(pirPin, INPUT);
  digitalWrite(pirPin, LOW);

  //give the sensor some time to calibrate
  for (int i = 0; i < calibrationTime; i++) {
    delay(1000);
  }
  delay(50);

  pinMode(pirPin, INPUT); // make PIR sensor an input
  digitalWrite(pirPin, LOW); // activate internal pull-up resistor
  if(!sd.begin(9, SPI_HALF_SPEED)) sd.initErrorHalt();
  if (!sd.chdir("/")) sd.errorHalt("sd.chdir");
  result = MP3player.begin(); // start mp3 player shield
  MP3player.setVolume(10, 10);
}

// loop
void loop() {
  if (digitalRead(pirPin) == HIGH) { // if movement sensed
    randomSeed (millis()); // set a more random seed for the random function
    rantrack = random(5);// find random number
    result = MP3player.playTrack(rantrack);// play track
    playtime();// call function for play time
    MP3player.stopTrack();// stop track
    delay(pauseTime);// wait…
  }
  delay(readingInterval); // wait with reading
}

// function to determine playtime
void playtime() {
  startingMillis = millis(); // set both time counter
  currentMillis = millis();
  while (currentMillis - startingMillis < playTime) { // while track plays, runs until playTime is reached
    currentMillis = millis(); // set to actual time
    delay(40); // debounce
  }
}