var vid = document.getElementById("myVideo");
alert(vid.currentSrc); 


document.getElementById("mp4_src").src = "movie.mp4";


<audio id="mijnAudio">
  <source src="mov_bbb.mp4" type="video/mp4">
  Your browser does not support HTML5 video.
</audio>

<script>
var aid = document.getElementById("mijnAudio");

function enableControls() { 
    aid.controls = true;
    aid.load();
} 


    document.getElementById("ogg_src").src = "movie.ogg";
    document.getElementById("myVideo").load();
} 

var vid = document.getElementById("myVideo");

function playVid() { vid.play(); }

<audio id="myAudio" controls autoplay>

<script>
function myFunction() {
    var x = document.getElementById("myAudio").readyState;
    document.getElementById("demo").innerHTML = x;
}
</script>



<audio id="myAudio" controls>
  <source src="horse.ogg" type="audio/ogg">
  <source src="horse.mp3" type="audio/mpeg">
  Your browser does not support the audio element.
</audio><br>

<button onclick="enableControls()" type="button">Enable controls</button>
<button onclick="disableControls()" type="button">Disable controls</button>
<button onclick="checkControls()" type="button">Check controls status</button>

<script>
var x = document.getElementById("myAudio");
function enableControls() { 
    x.controls = true;
    x.load();
} 

function disableControls() { 
    x.controls = false;
    x.load();
} 
 
function checkControls() { 
    alert(x.controls);
} 
</script> 

<video id="myVideo" controls autoplay>
  <source id="mp4_src" src="mov_bbb.mp4" type="video/mp4">



<script> 
function myFunction() { 
    document.getElementById("mp4_src").src = "movie.mp4";
    document.getElementById("ogg_src").src = "movie.ogg";
    document.getElementById("myVideo").load();
} 
</script>