I don't know about 'easier to hear', within a well produced piece of audio, you should be able to hear everything.
Within a chord though, for example, I associate the highest note of a chord as the 'one that stands out'.
I have made two short quick examples of "Twinkle, Twinkle, Little Star" to demonstrate what I mean. (I used the first online sequencer I could find, I'm really unfamiliar with it, I hope it works - I have a very basic setup so I hope they do. If not, I'm sorry - perhaps a project for meta might be to get something we can embed more easily into questions and answers, like how chess.stackexchange.com have an embeddable game playback viewer).
The first example, the tune is played with chords where the highest note of each chord also belongs to the melody: http://onlinesequencer.net/20687 (midi version) - and this sounds about right.
The second example, the tune is played with chords where the lowest note of each chord plays the main melody: http://onlinesequencer.net/20693 (midi version) - which, to me, is barely recognisable, and more suitable as a harmony/accompaniment for the melody, for the left hand.
(Un)scientifically, I think two frequencies of the same sound (so, same instrument) with the same amplitude will vibrate the same amount of air, for a single sound wave length (possibly not the same because the thing that produces the lower frequency is vibrating more slowly). A higher frequency sound has a shorter wavelength, so you would expect it to vibrate more air because it is moving at a faster rate. Whether this is compatible with the mechanics of hearing, I don't know, but within the pitch-range of a musical instrument, the higher notes of chords appear to be heard more prominently.