I bet this is a noobish question, but I've checked the forums and the manual and haven't found an answer to this.
You can set the voice mode like this:
SetVoiceMode(eSpeechVoiceAndText);
However, how do you get the voice mode?
Hmmm there doesn't seem to be a way.
You may use a global variable to keep this setting.
To expand on Gilbot's suggestion, I'd say to set both in game_start, and then use a custom function that sets the global var and calls SetVoiceMode().
~Trent