I can't hear the game sound effects on my iPhone/iPod Touch.
There is a toggle button on the side of your iPhone that silences the ringer and other tones. If this is set to silent it will silence the game effects.
The sound effects for the game are on the same channel as your music. So, while you can enable/disable the sound effects within the game, the only way to change the volume of the sound effects is the volume control for the main device.