Normally, adding the keycode to audio response stuff would allow you to accept a keypress while the voice rt is occurring. e.g., see:

www.empirisoft.com/Support/showthread.php?t=266

In your case, it sounds like you want a keypress response *after* the voice response in order to indicate whether or not it was correct. If so, I would just add this as a subsequent response--e.g., "~","0,0,1"."rt:57,43" where the first code indicates correct and the second indicates incorrect. Would that work for you?