Source code for each of the buttons

<script>
document.addEventListener("DOMContentLoaded", (event) => {
  // Event listener for Play button
  document.querySelector(".btn-play").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage("playerPlay", "*");
  });

  // Event listener for Pause button
  document.querySelector(".btn-pause").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage("playerPause", "*");
  });

  // Event listener for Stop button
  document.querySelector(".btn-stop").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage("playerStop", "*");
  });

  // Event listener for Mute button
  document.querySelector(".btn-mute").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage("playerMute", "*");
  });

  // Event listener for Unmute button
  document.querySelector(".btn-unmute").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage("playerUnmute", "*");
  });

  // Event listener for Fullscreen button
  document.querySelector(".btn-fullscreen").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage("playerFullscreen", "*");
  });

  // Event listener for Seek button
  document.querySelector(".btn-seek").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage({ type: "playerSeek", time: 3 }, "*");
  });

  // Event listener for Set Volume button
  document.querySelector(".btn-set-volume").addEventListener("click", () => {
    var iframe = document.querySelector('iframe');
    iframe.contentWindow.postMessage({ type: "playerVolume", volume: 0.3 }, "*");
  });
});
</script>