If I wanted to speech-enable my application, should I use VoiceXML or JavaSpeech?

It depends on what kind of application you are talking about. VoiceXML is really designed for web applications. It's another markup language - instead of displaying content on a screen, it uses speech and voice response to interact with the user (usually) over a phone. If you are writing a desktop application, then VoiceXML probably won't be suitable. If you want to embed voice commands to control your app, then JavaSpeech is probably the way to go.