I also have that version installed (used with Chrome) but have never
found any setting to turn its audio function on.
Firefox has its own Notifier version (which shows the "envelope" in
Red instead of Blue!) but as I use Chrome, that's not my field.
The version I use which allows me to be aurally notified is a Google
Gadget, and works in my sidebar. This notifies me of new arrivals, and
allows me to monitor various aspects of my PC without use of pop-ups.
I use the old AOL-sourced "You've got Mail" wav file to inform me when
I'm away from the screen, or using another workstation.