Such as noise, how good the bluetooth device is and more. I can't cure that but try a wired headset first to see if it's the usual bluetooth cheap thing issue.
Did you try other apps? Such as http://www.google.com/mobile/voice-actions/ ?
Did you test this out in a quiet office first?
How do I get voice commands when I am driving? I set my phone to driving mode using bluetooth. When I am making a call I get a series of bells and the phone displays questions. All this defeats the purpose of hands free communication. What do I need to do differently?