When the timer has completed counting down a message box appears and an audible beep can be heard, assuming you have the sound turned on. For some folks the beep just isn't enough. A.D.D. Timer has an additional feature that utilizes the text-to-speech engine built into Microsoft Windows in order to speek the message.
If you want your timer to speak the message for a timer simply check the "Speak the Message" check box when you create your launcher..
Depending on your version of Windows, you can change how the voice sounds in the Speech Option Control Panel in your Windows Control Panel. Look for the Text-To-Speech tab.