Hi there! Sorry for the late reply. Inside your config I can se an error. "letter_sound" is not an option for setting a custom sound letter, but only for enabling/disabling it (true or false). Right now, the plugin can't set custom letters' sound, but as It's a feature that has been requeasted by a lot of customers, I was planning to add it inside a quality of update of the plugin.
Right now I'm quite busy, but I was thinking to make some plugins' updates next week. Hopefully, I will make the update of this one, too :D