sound: fix sound-buffer-in-use issue

FGFX objects must be dereferenced early enough, and sound manager must be
removed late enough - otherwise openal complains about resources being
still in use when tryin to remove buffers.
Also: do not create FGFX objects for AI/MP aircraft, when AI sound is
disabled.

src/AIModel/AIBase.cxx

@@ -228,7 +228,7 @@ void FGAIBase::update(double dt) {
                             pitch*speed );
         _fx->set_velocity( velocity );
     }
-    else if (_aimodel)
+    else if ((_aimodel)&&(fgGetBool("/sim/sound/aimodels/enabled",false)))
     {
         string fxpath = _aimodel->get_sound_path();
         if (fxpath != "")
@@ -492,6 +492,8 @@ void FGAIBase::unbind() {
 
     props->setBoolValue("/sim/controls/radar/", true);
 
+    // drop reference to sound effects now
+    _fx = 0;
 }
 
 double FGAIBase::UpdateRadar(FGAIManager* manager) {

src/Main/globals.cxx

@@ -171,6 +171,7 @@ FGGlobals::~FGGlobals()
         ai->unbind();
         delete ai;
     }
+    SGSubsystem* sound = subsystem_mgr->remove("sound");
 
     subsystem_mgr->shutdown();
     subsystem_mgr->unbind();
@@ -205,6 +206,7 @@ FGGlobals::~FGGlobals()
     delete tacanlist;
     delete carrierlist;
     delete channellist;
+    delete sound;
 }
 
 

src/Model/acmodel.cxx

@@ -91,6 +91,9 @@ FGAircraftModel::reinit()
 void
 FGAircraftModel::deinit()
 {
+  // drop reference
+  _fx = 0;
+
   if (!_aircraft) {
     return;
   }