--- a/dom/media/tests/mochitest/head.js
+++ b/dom/media/tests/mochitest/head.js
@@ -7,32 +7,41 @@
 var Cc = SpecialPowers.Cc;
 var Ci = SpecialPowers.Ci;
 
 // Specifies if we want fake audio streams for this run
 let WANT_FAKE_AUDIO = true;
 // Specifies if we want fake video streams for this run
 let WANT_FAKE_VIDEO = true;
 let TEST_AUDIO_FREQ = 1000;
-let audioDevice = SpecialPowers.getCharPref("media.audio_loopback_dev", "");
-if (audioDevice) {
-  WANT_FAKE_AUDIO = false;
-  // It will be updated to 440 when/if DefaultLoopbackTone is instantiated.
-  TEST_AUDIO_FREQ = -1;
-  dump("TEST DEVICES: Got loopback audio: " + audioDevice + "\n");
-} else {
-  dump("TEST DEVICES: No test device found in media.audio_loopback_dev, using fake audio streams.\n");
+
+/**
+ * Reads the current values of preferences affecting fake and loopback devices
+ * and sets the WANT_FAKE_AUDIO and WANT_FAKE_VIDEO gloabals appropriately.
+*/
+function updateConfigFromFakeAndLoopbackPrefs() {
+  let audioDevice = SpecialPowers.getCharPref("media.audio_loopback_dev", "");
+  if (audioDevice) {
+    WANT_FAKE_AUDIO = false;
+    dump("TEST DEVICES: Got loopback audio: " + audioDevice + "\n");
+  } else {
+    WANT_FAKE_AUDIO = true;
+    dump("TEST DEVICES: No test device found in media.audio_loopback_dev, using fake audio streams.\n");
+  }
+  let videoDevice = SpecialPowers.getCharPref("media.video_loopback_dev", "");
+  if (videoDevice) {
+    WANT_FAKE_VIDEO = false;
+    dump("TEST DEVICES: Got loopback video: " + videoDevice + "\n");
+  } else {
+    WANT_FAKE_VIDEO = true;
+    dump("TEST DEVICES: No test device found in media.video_loopback_dev, using fake video streams.\n");
+  }
 }
-let videoDevice = SpecialPowers.getCharPref("media.video_loopback_dev", "");
-if (videoDevice) {
-  WANT_FAKE_VIDEO = false;
-  dump("TEST DEVICES: Got loopback video: " + videoDevice + "\n");
-} else {
-  dump("TEST DEVICES: No test device found in media.video_loopback_dev, using fake video streams.\n");
-}
+
+updateConfigFromFakeAndLoopbackPrefs();
 
 /**
  *  Global flag to skip LoopbackTone
  */
 let DISABLE_LOOPBACK_TONE = false;
 /**
  * Helper class to setup a sine tone of a given frequency.
  */
@@ -349,32 +358,37 @@ function createMediaElementForTrack(trac
 /**
  * Wrapper function for mediaDevices.getUserMedia used by some tests. Whether
  * to use fake devices or not is now determined in pref further below instead.
  *
  * @param {Dictionary} constraints
  *        The constraints for this mozGetUserMedia callback
  */
 function getUserMedia(constraints) {
+  // Tests may have changed the values of prefs, so recheck
+  updateConfigFromFakeAndLoopbackPrefs();
   if (!WANT_FAKE_AUDIO
       && !constraints.fake
       && constraints.audio
       && !DISABLE_LOOPBACK_TONE) {
     // Loopback device is configured, start the default loopback tone
     if (!DefaultLoopbackTone) {
       TEST_AUDIO_FREQ = 440;
       DefaultLoopbackTone = new LoopbackTone(new AudioContext, TEST_AUDIO_FREQ);
       DefaultLoopbackTone.start();
     }
     // Disable input processing mode when it's not explicity enabled.
     // This is to avoid distortion of the loopback tone
     constraints.audio = Object.assign({}, {autoGainControl: false}
                                         , {echoCancellation: false}
                                         , {noiseSuppression: false}
                                         , constraints.audio);
+  } else {
+    // Fake device configured, ensure our test freq is correct.
+    TEST_AUDIO_FREQ = 1000;
   }
   info("Call getUserMedia for " + JSON.stringify(constraints));
   return navigator.mediaDevices.getUserMedia(constraints)
     .then(stream => (checkMediaStreamTracks(constraints, stream), stream));
 }
 
 // These are the promises we use to track that the prerequisites for the test
 // are in place before running it.

--- a/dom/media/tests/mochitest/test_getUserMedia_basicAudio_loopback.html
+++ b/dom/media/tests/mochitest/test_getUserMedia_basicAudio_loopback.html
@@ -11,16 +11,17 @@
     title: "getUserMedia Basic Audio Test Loopback",
     bug: "1406350",
     visible: true
   });
   /**
    * Run a test to verify the use of LoopbackTone as audio input.
    */
   scriptsReady.then(() => runTestWhenReady(async () => {
+    let audioDevice = SpecialPowers.getCharPref("media.audio_loopback_dev", "");
     if (!audioDevice) {
       todo(false, "No loopback device set by framework. Try --use-test-media-devices");
       return Promise.resolve();
     }
 
     // At this point DefaultLoopbackTone has been instantiated
     // automatically on frequency TEST_AUDIO_FREQ (440 Hz). Verify
     // that a tone is detected on that frequency.