I like your game a lot! But i have some ideas for the game.
1. Make buttons to response some other way than only by sound. Maybe enlarge them or change their color/appearance(witch i think is the best way)
2. It would be nicer if you could put navigation buttons to bottom of the screen. Now i need to hold my phone weirdly or use my another hand(it's physically demanding thing <img src="smileys/smiley4.gif" border="0" align="middle" />)