HTML video has controls, but doesn't load video or play - html

Question

This is for a school assignment but I can't seem to get the video to work.
I'm using chrome. The buttons load and so does the player(black screen), but I can't get it to seem to work.
<div style="text-align:center">
<button onclick="playPause()">Play/Pause</button>
<button onclick="makeBig()">Big</button>
<button onclick="makeSmall()">Small</button>
<button onclick="makeNormal()">Normal</button>
<br/>
<video id="myVideo" width="420">
<source src="audi.mp4" type="video/mp4">
</video>
</div>
<script>
var myVideo = document.getElementById("myVideo");
function playPause() {
if (myVideo.paused)
myVideo.play();
else
myVideo.pause();
}
function makeBig() {
myVideo.width = 560;
}
function makeSmall() {
myVideo.width = 320;
}
function makeNormal() {
myVideo.width = 420;
}
</script>