FreeChat for WhatsApp

I've got WhatsApp on my phone, and 'FreeChat for WhatsApp' on my MacBook Pro.

It works well with both.
The only thing I would like to disable is the sound when a new message arrives.
Of course, on my phone it was not difficult, but I didn't find out yet how to do that on my MacBook.
In "Notifications" it is disabled, but nevertheless I get that "Ping!" every time...