I have the same question for The Bat v5.8.8. Sound notification works for my POP3 accounts, but I can't even find an option or setting for the IMAP account.
Well, that works, but instead of playing the sound once when I receive a bunch of new emails all at once, it plays the sound once for each new email. If I get 16 new emails, it sits there and plays the notification sound 16 times, which is kind of ridiculous.
I really love The Bat! and this is the only very little nuisance that I always thought could use fixing. Since I was really bored the other day I wrote myself a program that will play a notification sound that can be set up as an incoming filter. It addresses the problem Warner mentioned so it will only play the notification sound once if you get multiple new emails.
To set it up, create a new filter for any message and run it as external action like so