Must to have sound. Something like Tetris I can tolerate without sound, but I'd rather hear the game than play silently. I mean, the sound is as much part of the experience as the graphics or gameplay, in my opinion. Though sometimes a certain part of the sound can be turned off......say like the music in Zoo Keeper, for example. Other times, the sound needs to be adjusted to make the balance of music and sound effects better. But NO SOUND? Nah, can't deal with that in most cases.
I voted "ON".