Most of the speech hubbub around Mountain Lion has centered on the OS's marquee Dictation feature, which happily accepts your spoken words as a substitute for typing them in. Dictation works in almost any text entry field, and it's surprisingly effective; Steve even dictated his entire post about Dictation.
Speech-to-text is only one side of the coin; there's also text-to-speech. OS X Leopard introduced a single high-quality voice named Alex. "He" sounded so natural compared to the previous generation of Mac synth voices that it was a little bit disconcerting.
Starting in OS X Lion, users were given the choice to install high-quality synthetic voices licensed from Nuance that supplemented or replaced the "classic" Mac voice options in scores of languages. These voices delivered uncanny quality while chewing up hefty amounts of disk space (upwards of 500 MB in some cases).
As pointed out by AppleInsider, the enhanced speech voices have now been updated for Mountain Lion. Users who previously installed a custom voice should now see 2.0 versions of those voices available in the new Software Update zone (which appears at the top of the Updates area in the Mac App Store). If you've never experimented with the voice synth options in OS X, you can change the system voice in the Dictation & Speech system preference pane.
Want to make your Mac speak? TextEdit (and most Cocoa-based editors) offers Speech options in the Edit menu or via a contextual menu. You can also pick a hotkey in the Dictation & Speech preference pane to speak any selected text in any application. The preference pane also allows you to turn on spoken alerts for notifications, speak the time or let you know when an app needs your attention, which I imagine would grow tiresome awfully quickly.
OS X's voice synthesis skills are also put to full use with VoiceOver, the accessibility screenreader that assists visually impaired users. Sighted users may find it's worth experiencing VoiceOver once or twice, just to get a sense of the amount of engineering work that goes into making OS X a true accessible platform. To turn on the full screenreader interface with VoiceOver, you can use the Accessibility preference pane or just hit ⌘-F5.
Automator and AppleScript both support speech output, and there's still the venerable say command-line tool which lets you specify a voice with the -v flag. If you're feeling particularly mischief-minded, remember that say will work on a remote machine via an ssh session. Watch as your officemate jumps clear out of his or her skin when you tell the MacBook Pro on the desk to
say -v Trinoids Do not adjust your screen. You will be assimilated. The process is quick and painless, stand by.
Fun for the whole family.