General Q and A

As I'm sure you can ask any physics professor, the human ear has a logarhytmic perception of sound. That's why we measure sounds, when the exact energy content is not important, in decibeliums. They're a measure of the logarhytm of the amount of energy per square metre, adjusted by the frequency of the sound- but let's ignore that part for now.
But here's the keyword- logarhytm. Assuming a person 1000 times smaller (in mass) could create a sound with a thousandth part of the energy, that means the sound is only 30 decibeliums lower. Assuming a person were a million times smaller and a direct proportion between mass and energy, you'd get a sound 60 decibeliums lower (10 decibeliums are one decimal order of magnitude). Assuming a person were 468 million times smaller, you'd get a sound around 82 decibeliums lower.

BUT, that'd be assuming Milly can't hear anything under 41 decibeliums, because it takes around 10000 times the noise for her to be able to discern it. She ain't deaf! If her hearing is at least as good as yours, then she should be able to hear anything you should- meaning a net loss of only 41 decibeliums. A yell can reach 90 decibeliums, which, minus 41, would be 49 decibeliums. Roughly the sound of a crowd, or enough to attract her attention.

From there, use the square/linear law: the energy carried per unit of surface in a soundwave is inversely proportional to the square of the distance. So at half the distance, you hear it with four times the energy- and at 1/8 of the distance, you hear it with roughly 1000 times the energy, or almost 10 decibeliums higher.

And I dunno if you should trust a direct proportion between sound volume and size either. There's always those tiny effin' insects that make enough ruckus to drive ya to manslaughter, after all!