A quick search of SourceForge.org does not show "VoiceBox" yet

I assume you're using the SAPI SDK? What version 5.X? Personally I'm hoping its 5.3, but that would limit it to Vista only. I've tested a few apps using earlier version than 5.3 and all had/have some minor/major issues with Vista. I was reading the MSDN on SAPI 5.3 last night and I'm toying with the idea of pulling out VS and having a play with it again - its been "a few years" since I did anything serious with VS.