As others have pointed out, this was a poorly conceived poll, since almost no one does just one thing (even in general.)
I usually play with no sound when I'm on the bus or walking around the city. Occasionally I use headphones in those situations for games that require audio (like Theatrhythm FF.) At home, I play with the system sound usually, although occasionally I'll play with headphones to get the best audio experience. Sometimes I even use a pair of powered speakers or a Slime speaker.
With my PS Vita, I use wireless bluetooth stereo headphones or a mono headset. I wish they had bothered to include a bluetooth radio in the 3DS so that we could use bluetooth headphones and headsets on it, too. If WiiU doesn't support Bluetooth either, that will be pretty depressing.