--- a/dom/media/webaudio/test/test_mediaElementAudioSourceNodeCrossOrigin.html
+++ b/dom/media/webaudio/test/test_mediaElementAudioSourceNodeCrossOrigin.html
@@ -34,24 +34,34 @@ var testsRemaining = tests.length;
 
 tests.forEach(function(e) {
   e.ac = new AudioContext();
   var a = new Audio();
   if (e.cors) {
     a.crossOrigin = e.cors;
   }
   a.src = e.url;
-  var measn = e.ac.createMediaElementSource(a);
-  var sp = e.ac.createScriptProcessor(2048, 1);
-  // Set a couple expandos to track the status of the test
-  sp.seenSound = false;
+  document.body.appendChild(a);
 
-  measn.connect(sp);
-  a.play();
-  document.body.appendChild(a);
+  a.onloadedmetadata = () => {
+    // Wait for "loadedmetadata" before capturing since tracks are then known
+    // directly. If we set up the capture before "loadedmetadata" we
+    // (internally) have to wait an extra async jump for tracks to become known
+    // to main thread, before setting up audio data forwarding to the node.
+    // As that happens, the audio resource may have already ended on slow test
+    // machines, causing failures.
+    a.onloadedmetadata = null;
+    var measn = e.ac.createMediaElementSource(a);
+    var sp = e.ac.createScriptProcessor(2048, 1);
+    sp.seenSound = false;
+    sp.onaudioprocess = checkBufferSilent;
+
+    measn.connect(sp);
+    a.play();
+  };
 
   function checkFinished(sp) {
     if (a.ended) {
       sp.onaudioprocess = null;
       var not = e.expectSilence ? "" : "not";
       is(e.expectSilence, !sp.seenSound,
          "Buffer is " + not + " silent as expected, for " +
          e.url + " (cors: " + e.cors + ")");
@@ -72,15 +82,13 @@ tests.forEach(function(e) {
     }
     // It is acceptable to find a full buffer of silence here, even if we expect
     // sound, because Gecko's looping on media elements is not seamless and we
     // can underrun. We are looking for at least one buffer of non-silent data.
     e.target.seenSound = !silent || e.target.seenSound;
     checkFinished(e.target);
     return silent;
   }
-
-  sp.onaudioprocess = checkBufferSilent;
 });
 </script>
 </pre>
 </body>
 </html>