The voice of a Mac can come from two sources. One is the VoiceOver utility, which is part of the standard accessibility features, and the other is the alert notification in the Dictation and Speech category of System Preferences. VoiceOver can be disabled two ways. Both voice sources are disabled by default. However, if they are enabled and you want to turn them off, it takes only a few steps. Note that the information in this article applies to OS X Mavericks. It may vary with other versions or products.
The first way to turn off the VoiceOver function is to press the "Command" and "F5" keys simultaneously. The VoiceOver message box displays "VoiceOver Off," and the Mac voice says "voiceover off." After that, there are no more voice messages from VoiceOver. Press "Command" and "F5" simultaneously again to toggle VoiceOver back on.
You can also turn off VoiceOver using System Preferences. Click the "Apple" icon on the menu bar. Choose "System Preferences" from the drop-down menu. Choose "Accessibility." Click "VoiceOver" on the following screen and click "Disable VoiceOver."
Dictation and Speech Alert Announcements
Click the "Apple" icon on the menu bar and choose "System Preferences" from the drop-down menu. Choose "Dictation & Speech" and then click "Text to Speech." Uncheck the box next to "Announce when alerts are displayed." If desired, set more options by clicking "Set Alert Options." Close the Dictation & Speech box when you are finished.