Unfortunately the Atmega328 isn't powerful enough to run a voice recognition engine... but would not be hard to make it responds to claps or loud single sounds (again I'm not sure if I recommend this because you certainly don't want to turn this one when it's in your bag for example).
However, you are tempting me to throw in an ARM Cortex M4 on the flashlight and run a voice recognition engine on board... there's certainly enough space ;)