Only enable sound detection if supported

Also use a lower threshold. -80 should be enough
And should Fix #55

public/js/vroom.js

@@ -1012,14 +1012,16 @@ function initVroom(room) {
   // When we joined the room
   webrtc.on('joinedRoom', function(){
     // Check if sound is detected and warn if it hasn't
+    if (window.webkitAudioContext || window.AudioContext){
       setTimeout(function (){
-      if (maxVol < -60){
+        if (maxVol < -80){
           $.notify(locale.NO_SOUND_DETECTED, {
             className: 'error',
             autoHide: false
           });
         }
       }, 10000);
+    }
     // Notify the server a new participant has joined (ourself)
     // If we were prompted for our display name before joining
     // we send it. Not that I like sending this kind of data to the server