Text-to-Speech (voice) support is not enabled in nightly builds. Visual Studio Express editions can't build with it enabled, so we have it disabled in the project file by default. It will be in Release Candidates and Final builds, and should work as well as it has in the past. But that's why we have release candidates before a final build :)
Text to Speech and Voice Recognition have both been enabled by default in the VS 2013 and 2015 project files so the nightlies have had these enabled for some time.
Should I download "standard" or "Inferno"?
Inferno - keep in mind that it will update your pilot files if you put them in the inferno pilots folder, making them unusable on non-inferno builds when placed in the regular pilots folder. So copy them, don't move them if you want to use the pilot on both for now. The Inferno differences will be made standard in the near future.
All Inferno changes have now been unified as 'Standard'.