Why is his singing heard on a "distant hill"?

The answer in this case, as it is with most poems, really depends on the way you choose to interpret it. That's not to say that every interpretation has a sound basis in the work itself, but the symbols of Maya Angelou's "Caged Bird" don't offer an easy, singular answer. I'd offer you a few ideas and musings on what it could mean.
For one, as you read the poem you might notice that the free bird never sings. It "leaps" and "floats" and thinks of food, but it does not sing. Every reader is welcome to disagree, but personally I don't think it's accidental on the author's part. So why does the free bird not sing? It's possible he doesn't need to. Maybe he doesn't want to, or perhaps he's forgotten how to. In contrast, the caged bird does sing. That's pretty much all he can do, because:

his wings are clipped and his feet are tied

So why is the caged bird's song heard on "a distant hill?" It's very likely that it has to do with desperation and passion. The song is everything the caged bird has; freedom is all he thinks about. The "bars of rage" stand between him and everything the free bird experiences. The caged bird doesn't even know what he is missing, because he sings:

of things unknown
but longed for still

We can imagine that kind of longing gives the song the power to be heard far away on the distant hill. Now this could mean that the song itself is powerful, but it could also mean that the message itself is. After all, Angelou says that the song is heard:

for the caged bird
sings of freedom

So we could interpret that as saying it's heard because he sings of freedom. It's possible the message is that freedom songs carry further than others. After all, most of us are free, but we can imagine being trapped and wouldn't wish that fate on others. The free bird isn't just symbolic of being free—he's also symbolic of being a bird. He does the things a bird is supposed to do, while the caged bird is trapped in an unnatural state. Therefore you could say that the caged bird's song is heard, since we care to hear to hear it.

The poem doesn't specify exactly why this is the case, but we can interpret that, in the context of the poem, this distant hill represents the freedom which the caged bird longs for, but has yet to achieve. The idea of his singing being heard on this hill, then, could be understood in more than one way. First, we might assume that those who have managed to make their way to this distant hill are others who have escaped oppression ahead of the caged bird and now hear the singing of their trapped fellows who are longing to break out—the implication being that those who have freed themselves should always be listening to others who are still in need of help. We could also understand this comment to be a rumination on the sheer power and constancy of the caged bird's cries as it yearns for freedom.