The Software License Agreement for Mac OS X 10.7.3 suggests that it is legally NOT OK to us the System Voices for anything else than your personal, non-commercial use:
"G. 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."
See http://www.apple.com/legal/sla/