Sadly I don't have a perfect memory, so I don't actually know whether there are scenes supporting this theory, but I thought:
Seems like any time balloons are shown in MLP, they float up, i.e. they are helium balloons. ... Do you know of any scene where Pinkie Pie inflates a balloon with her lungs that ends up floating upwards? If not, then I guess the writers really thought it through. ... Or not, haha. We're talking about Pinkie here. I would find it quite natural if her lungs expelled helium. It would be like the physical manifestation of her overall lightness.
A partial helium content in her breath would also explain her cute voice. ^^
And yours. Admit it: If Pinkie was all in-your-face talking to you, your voice would get squeaky, too.
It could of course also be that she's simply breathing out magic.