Wrote a test which exercises audio-only WebRTC calls.

content/test/data/media/webrtc_test_audio.js

Index: content/test/data/media/webrtc_test_audio.js
diff --git a/content/test/data/media/webrtc_test_audio.js b/content/test/data/media/webrtc_test_audio.js
index 0cf4ac7f62a000caa7077de79c34e4357b72df40..72760cf87d4a27311f05cc36e1135ca38f6afe4e 100644
--- a/content/test/data/media/webrtc_test_audio.js
+++ b/content/test/data/media/webrtc_test_audio.js
@@ -11,11 +11,19 @@ var MAX_AUDIO_OUTPUT_ENERGY = 32768;
 // calls back |callback| with an array with numbers in the [0, 32768] range.
 function gatherAudioLevelSamples(peerConnection, numSamples, frequency,
                                  callback) {
-  console.log('Gathering ' + numSamples + ' audio samples...');
+  console.log('Gathering ' + numSamples + ' audio samples (waiting for call ' +
+              'to get up if necessary)...');
   var audioLevelSamples = []
   var gatherSamples = setInterval(function() {
     peerConnection.getStats(function(response) {
-      audioLevelSamples.push(getAudioLevelFromStats_(response));
+      audioOutputLevels = getAudioLevelFromStats_(response);
+      if (audioOutputLevels.length == 0)
+        return;  // The call probably isn't up yet.

[no longer working on chromium, 2014/06/05 09:12:32]

nit, make it look more chromium:
if (audioOutputLevels.length) {
  // The call probably isn't up yet.
  return;
}

[phoglund_chromium, 2014/06/09 06:55:36]

Done.

+
+      // If more than one audio level is reported we get confused.
+      assertEquals(1, audioOutputLevels.length);
+      audioLevelSamples.push(audioOutputLevels[0]);
+
       if (audioLevelSamples.length == numSamples) {
         console.log('Gathered all samples.');
         clearInterval(gatherSamples);
@@ -83,8 +91,5 @@ function getAudioLevelFromStats_(response) {
       audioOutputLevels.push(report.stat('audioOutputLevel'));
     }
   }
-  // Should only be one audio level reported, otherwise we get confused.
-  assertEquals(1, audioOutputLevels.length);
-
-  return audioOutputLevels[0];
+  return audioOutputLevels;
 }