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": "Reports if the engine is currently speaking. Note: reports if any engines system wide is speaking when using the native Mac OS X engine.",

[dmazzoni, 2012/04/23 23:50:46]

how about this:
Note: On Mac OS X, this will return true if the system speech engine is
speaking, even if it 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()."
       }
     ]
   }
 ]