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/UrdfParser.h

@@ -5,6 +5,7 @@
 #include "LinearMath/btAlignedObjectArray.h"
 #include "LinearMath/btHashMap.h"
 #include "URDFJointTypes.h"
+#include "SDFAudioTypes.h"
 
 #define btArray btAlignedObjectArray
 #include <string>
@@ -140,6 +141,8 @@ struct UrdfLink
 	
 	URDFLinkContactInfo m_contactInfo;
 
+	SDFAudioSource m_audioSource;
+
 	UrdfLink()
 		:m_parentLink(0),
 		m_parentJoint(0)