plumb URDF/SDF audio_source into PhysicsServerCommandProcessor, allow to play sounds on collision !

See also https://youtu.be/eppOjTfx5Jg for a first test, and this modified URDF how to add sounds:
https://github.com/bulletphysics/bullet3/blob/master/data/plane_with_collision_audio.urdf
Add the --audio flag to enable sound in pybullet/Bullet-C-API

examples/Importers/ImportURDFDemo/MyMultiBodyCreator.h

@@ -4,6 +4,7 @@
 
 #include "MultiBodyCreationInterface.h"
 #include "LinearMath/btAlignedObjectArray.h"
+#include "LinearMath/btHashMap.h"
 
 struct GUIHelperInterface;
 class btMultiBody;
@@ -28,7 +29,6 @@ protected:
     
 	struct GUIHelperInterface* m_guiHelper;
 
-	
 	btAlignedObjectArray<btGeneric6DofSpring2Constraint*> m_6DofConstraints;
 
 public: