How to get one audio track to stop playing when another starts playing (Minim)

I have two soundtracks- one named rain and another named heartbeat. The goal is to trigger these audio files when a key is pressed. My problem is that I need to ensure that when one is playing the other stops, and for some reason processing isn't recognizing the .stop() or .pause() functions.

I'm only a beginner with code, so if anyone can let me know what I'm doing wrong/how I can fix this, I'd appreciate it a lot!

    import ddf.minim.spi.*;
    import ddf.minim.signals.*;
    import ddf.minim.*;
    import ddf.minim.analysis.*;
    import ddf.minim.ugens.*;
    import ddf.minim.effects.*;

    Minim minim;
    AudioPlayer heartbeat;
    AudioPlayer rain;

    void setup()
    {
      size(400, 600);
      minim = new Minim(this);
      heartbeat = minim.loadFile("heartbeat.mp3");
      rain = minim.loadFile("rainforest.mp3");
    }

    void draw() {
      background(0);
      stroke(255);
    }

    void keyPressed() {
      if (key == 'h') {
        heartbeat.rewind();
        heartbeat.play();
        if (key == 'p') {
            heartbeat.pause(); }
      }
       if (key == 'r' && !rain.isPlaying()) {
        rain.rewind();
        rain.play();
        if (key == 's') {
            rain.pause(); }
      }
    }