[MlMt] new mail notification sound

[Baron Fujimoto]

As far as I can tell, MailMate should not configured to play any sounds
when new mail arrives, yet whenever a new message arrives in the INBOX
folder of my source, the "glass" sound plays.  The only place I could find
that seemed relevant was Preference > Counters > Sound. Am I missing
correct place to turn this off somewhere?