I think the windows 8 downgrade will only get you to vista. XP is not supported anymore. That being said, you could install XP if your hardware is carefully selected so that you will have proper device drivers. (or disable hardware that is not supported)
I would look to getting updated voiceprint software or maybe installing a virtual machine and run windows XP on it with the voice print software. kind of depends on the requirements of your software.
look at this link (run win 8 hyper-v with win xp virtual machine) http://www.techrepublic.com/blog/window-on-windows/install-windows-xp-in-windows-8-client-hyper-v/6732