Agh! I thought I would add to the already great solutions that, unfortunately, didn't work for me. I'm convinced this game is possessed and will only register lies for certain people!
The only thing that I found to consistently work...for me...is whistling loudly directly into the microphone.
I tried screeching, monotones talking, scratching, varied pitch and tone and nothing but whistling worked. I think it only registered like 11 or 12 lies the entire time I played the game.
Other than that, this was a kind of fun and very easy game for the 1K. (about 3 hrs) Thanks to the Achievement Hounds and their Viral Games for letting me borrow it.
Hope the whistling works and saves people some of the frustration that I experienced!