My perspective is a little different from those who have already commented. "Tiu" is the better choice in this sentence. The iu-correlatives are used when mentioning one item (or a comparatively small number of items) out of a closed set. "Tiu estas aparta voĉo" calls to mind a discussion about a number of voices under consideration - and pointing out one as standing out somehow.
With "tio" (tio estas aparta voĉo, ĉu ne") no clear context springs to mind. "Tio" generally answers the question "Kio" which is in turn generally used when you don't know what something is. So with "tio" we imagine someone asking "what is that" and the answer being "that is a particular voice, isn't it?" It's a very odd thing to say, at the very least.