The reason that there is no closing sound on the first enderchest is that my plugin can use the default one. If you put the key useVanillaEnderchest to false in the configuration file, you will see that the bug no longer occurs. (more info here)

And I can't handle the event properly with the default enderchest... However I will try to do something.

okay, thanks!

Now this should work for all enderchests (even the default one) thanks to this commit: 65609ee
Hope that it brings the behavior that you want.

Thanks!