Blow up several balloons to different sizes. If you have an elongated balloon, before blowing it up, slide it into a washer. Once the elongated balloon is inflated, it will have a noticeably strange shape since the washer is constricting the middle. Hold each balloon up to your ear and tap the other side of the balloon. Notice the differing volume levels of the taps between the different balloons, even among balloons that are the same in every way, except the amount of air blown into them.
Inflate several balloons to roughly the same size. Fill one up with helium, another with CO2 and another by just blowing it up. (You can fill a balloon with CO2 by putting dry ice into it. But make sure to start with small amounts of dry ice to ensure you don't burst the balloon.) Place a radio at one end of the room you are in and stand at the other end with all of the balloons. Have the radio turned on so that you can hear it, but not too loud. Turn your head and point one ear toward the radio. Hold each balloon up to your ear and listen to the radio sound come through the balloon, noticing the differing volumes. Carbon dioxide is denser than air, which in turn is denser than helium. The molecules being more dense mean the gases will be better conductors of sound, so the CO2 balloon should amplify the sound the most, followed by air, then helium.
Blow up several balloons. For this experiment, you can also fill a few balloons with helium. Hold up the balloons to your ear and have a friend press their lips against the other side of the balloon and whisper. The whisper should be louder now than without the balloon, as you have compressed the air molecules into the balloon by filling it, thus making it a better conductor of sound. Place balloons into the freezer. (Keep in mind while inflating the balloons that they will need to fit in the freezer. You can also remove some things from the freezer, as this part of the experiment takes only a few minutes.) Keep an eye on the balloons as they cool off. Due to the slowing of these molecules, the balloons will shrink. Once the balloons have become smaller, remove them from the freezer and once again hold each balloon against your ear and let a friend whisper into them. Note how well you can hear the whisper at this point, and then repeat this process every few minutes as the balloon warms. The balloon should conduct sound better when the balloon is colder because of the molecules being compressed while colder.
Take a tin coffee can and remove the bottom with a can opener. Cut off the bottom of two balloons where you would usually blow them up. Stretch each balloon over an end of the coffee can. Due to the elasticity of the balloons, they should stay in place. Take one to three rubber bands and stretch them around the coffee can, lengthwise. This should make it to where there is one rubber band, or more, stretched over each balloon on each end of the can. Pull the rubber bands and snap them, which will produce a drum effect that should be amplified within the can. Each rubber band should make a differing sound as well by being on different parts of the balloon. You can also remove one balloon from the coffee can and point that end of the can toward a friend. The sound will be more directed toward your friend and he should hear it louder than before, depending on your friend's distance.