defined default hotkey for sound on/off

docs/index.html

@@ -1518,8 +1518,8 @@
 
     <tr>
       <td>Toggle sound on/off (*)</td>
-      <td>Alt + '</td>
-      <td>Cmd + '</td>
+      <td>Control + ]</td>
+      <td>Control + ]</td>
     </tr>
 
     <tr>

src/common/PKeyboardHandler.cxx

@@ -524,6 +524,10 @@ void PhysicalKeyboardHandler::handleEvent(StellaKey key, StellaMod mod, bool sta
           myOSystem.console().changeHeight(-1);
           break;
 
+        case KBDK_RIGHTBRACKET: // Ctrl-] toggles sound
+          myOSystem.sound().toggleMute();
+          break;
+
         default:
           handled = false;
           break;