Uses a system-wide notion of isSpeaking in the Mac extension TTS api.

chrome/common/extensions/api/tts.json

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 [
   {
     "namespace": "tts",
     "types": [
       {
         "id": "TtsEvent",
@@ skipping 165 matching lines @@
       },
       {
         "name": "stop",
         "type": "function",
         "description": "Stops any current speech.",
         "parameters": []
       },
       {
         "name": "isSpeaking",
         "type": "function",
-        "description": "Checks if the engine is currently speaking.",
+        "description": "Checks whether the engine is currently speaking. On Mac OS X, the result is true whenever the system speech engine is speaking, even if the speech wasn't initiated by Chrome.",
         "parameters": [
           {
             "type": "function",
             "name": "callback",
             "optional": true,
             "parameters": [
               {
                 "name": "speaking",
                 "type": "boolean",
                 "description": "True if speaking, false otherwise."
@@ skipping 33 matching lines @@
             "name": "event",
             "$ref": "TtsEvent",
             "description": "The event from the text-to-speech engine indicating the status of this utterance."
           }
         ],
         "description": "Used to pass events back to the function calling speak()."
       }
     ]
   }
 ]