Google highlighted a Google voice search feature on Google+ the other day, which demonstrates how you can correct mistakes when Google doesn't understand what you are saying.
In the demonstration, the searches said they want artists "Baroque" but Google came back with broke artists instead. The searcher went back to Google and said "no, I said Baroque" in his voice and it figured it out on the second try.
Here is a picture, following by a video:
Forum discussion at Google+.