I want to use an excerpt of an OS X text-to-speech voice in a song that I am recording. Is there any kind of copyright restriction for recordings of the computer voice I should know about, or are they totally free for anyone to use for any purpose?
You are not free to use them for any purpose, only for personal, non-commercial use while running Apple software. Read Mac OS X Lion Software License Agreement for full details (section G, it is rather short and clear):
Voices Subject to the terms and conditions of this License, you may use the system voices included in the Apple Software (“System Voices”) (i) while running the Apple Software and (ii) to create your own original content and projects for your personal, non-commercial use. No other use of the System Voices is permitted by this License, including but not limited to the use, reproduction, display, performance, recording, publishing or redistribution of any of the System Voices in a profit, non-profit, public sharing or commercial context.