Corrected entry: Leonard and Penny are inhaling Sulphur Hexaflouride to make their voices low. The effect can be seen to be fake for two reasons. First, their voices go back to normal instantly and on demand - the gas actually takes a while to clear. Second, when Leonard releases the balloon, it flies away; it should just fall to the floor because Sulphur Hexaflouride is heavier than air.
Moose
Correction: 1. Wrong episode. This scene is from Episode 23. 2. Your voice will clear in as much time as it takes to clear your lungs - one or two breaths. 3. A sulfur hexafluoride balloon will still fly, as the escaping gas is still propelling it, as demonstrated in this clip by Mythbuster Adam Savage http://www.youtube.com/watch?v=d-XbjFn3aqE.
LorgSkyegon