Avoid nuisance sounds in initial update loop.

Stop all "in-transit" sounds effects from triggering in the first update
loop when their initial property value is "1".

simgear/sound/xmlsound.cxx

@@ -68,7 +68,8 @@ SGXmlSound::SGXmlSound()
     _dt_play(0.0),
     _dt_stop(0.0),
     _delay(0.0),
-    _stopping(0.0)
+    _stopping(0.0),
+    _initialized(false)
 {
 }
 
@@ -298,6 +299,13 @@ SGXmlSound::update (double dt)
    if (_property)
        curr_value = _property->getDoubleValue();
 
+   if (!_initialized)
+   {
+       // update initial value before detecting changes
+       _prev_value  = curr_value;
+       _initialized = true;
+   }
+
    // If a condition is defined, test whether it is FALSE,
    // else
    //   if a property is defined then test if it's value is FALSE

simgear/sound/xmlsound.hxx

@@ -149,12 +149,12 @@ private:
   double _dt_play;
   double _dt_stop;
   double _delay;        // time after which the sound should be started (default: 0)
-  double _stopping;	// time after the sound should have stopped.
-			// This is useful for lost packets in in-transit mode.
+  double _stopping;     // time after the sound should have stopped.
+                        // This is useful for lost packets in in-transit mode.
+  bool _initialized;
 
   std::vector<_snd_prop> _volume;
   std::vector<_snd_prop> _pitch;
-
 };
 
 #endif // _SG_SOUND_HXX