index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Game intro</title>
    <link rel="stylesheet" href="introStyle.css">
</head>
<body>
<section id="container-intro">
    <div id="clipped-title1">
        <h1>Wild card</h1>
    </div>
    <a href="fight.html" onmouseover="heartbeat()" onmouseleave="heartbeatStop()" class="button">PRESS START</a>
</section>

<footer>
    <p>&copy;2018 Wild Card</p>
</footer>

<script>

    const thunder = new Audio('music/Thunder.mp3');
    thunder.volume = 0.2;
    thunder.play();

    const audio = new Audio('music/heartbeat-03.mp3');
    const heartbeat = () => {
        audio.play();
        thunder.loop = false;
    }
    const heartbeatStop = () => {
        audio.pause();
        audio.currentTime = 0;
        thunder.play();
    }
</script>

</body>
</html>