NullPointerException using minim library

When I run my file i'm getting this NullPointerException error:

==== JavaSound Minim Error ====

==== java.io.FileNotFoundException: background.mp3

=== Minim Error ===

=== Couldn't load the file background.mp3

I have declared the audioplayer and also defined minim. The file is also in the correct folder as mp3. I can't understand at all why it shouldn't work. The code for the audio file is below. Any help would be much appreciated, thanks

This is the code for the audio:

import ddf.minim.*;

AudioPlayer player;

Minim minim;

void setup() {

size(800, 800);

background(0, 0, 0);

//p2 play sound file

//Construct a new instance of minim

minim = new Minim(this);

//Load the file we want to play, 2048 is the buffer size

player = minim.loadFile("background.mp3", 2048);

//play the file

player.play();

}