What you Need
What to doBlow across the bottle, remember the note it makes. Squash the bottle Blow across it again. Is there a difference in the note? Fill your container about 3/4 full of water See if immersing the bottle with water affects the note it makes, when it is both squashed and unsquashed. What may happenYou should find that squashing the bottle makes the note go down. When the bottle is immersed it will increase the pitch of the squashed bottle, but not the unsquashed one.
Why does it happen?
If a plastic bottle is flattened the changes in pressure cause the bottle can change shape slightly, if you touch the flattened area while you are blowing you can feel this as a strong vibration. This change in shape allows more air to flow in for the same change in pressure, so the speed of the vibration and therefore the pitch is lower than it should be for the amount of air available. If you surround the bottle with water by immersing it, the water effectively stops the walls from moving. This means that the pitch is correct for the amount of air in the bottle - which is of course reduced from its unsquashed volume so the pitch is higher than normal.
If the bottle is unsquashed on the other hand, because it is circular it can't get any larger without more plastic, and because there are no weak spots it is hard for it to get smaller. This means that the walls do not vibrate in the same way as in the squashed bottle, and putting the bottle in water will make no difference to the sound.
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When you blow over the top of the bottle the stream of air you produce can either be deflected into or out of the bottle. If it starts off going into the bottle the pressure will build up until the stream is pushed out, and then as the bottle empties the pressure drops and the stream can move back in. This produces a vibration you hear as sound. As you can find from the 
