updated 12:49 pm EDT, Fri July 27, 2012
Accents, different languages optional
Apple is now distributing v2.0.0 updates for some of the optional voices in OS X, notes AppleInsider. If not already loaded, the voices can be installed by clicking "Customize" in OS X Mountain Lion's upgraded Dictation & Speech panel. If an update is needed, Apple is currently distributing the v2.0.0 files via Software Update. Each download is described as delivering "pronunciation and clarity improvements."
By default OS X ships with a single English voice, Alex. The optional ones cover a range of different languages and accents, such as Irish, Scottish, and South African variants of English. Each voice is, however, a 1GB download.