We hear this tip all the time if we write. It may be the most crucial tip . . .
And I didn't listen for about five years. I read other people's work aloud, especially poems that rhymed, and I read out my own stuff when I got the chance, but in terms of performing I tend to get in over my head. Reading out loud to others? Definitely. Reading out loud to myself? NEVER.
Why this negativity? It stemmed from when I entered a gifted program where we followed books on audiotape. With respect to my Language Arts teacher, since in his class I learned to love reading and developed my love for horror, audio works didn't work for me. For one thing, I could read faster than I could listen to the stuff; I finished A Wrinkle in Time five days ahead of schedule. For another, I believed in characters having different voices; now I know that the speaker's quality is much more important. If you have two voices doing about ten characters, you were dead to me as a fourth and fifth grader.
But now, having received about a hundred rejections total for all the work I have, and writing poetry for class, a habit I haven't kept unfortunately, I've realized how important word flow needs to sound. We may see the letters on paper, but we also have to hear them. And if your readers even in the classroom don't laugh when you want them to, then something is wrong with the words, not with their hearing.
Even better, reading aloud will give each of our characters a voice. Neil Gaiman at Mouse Circus gives the Man Jack a terrifying growl, while the Sleer truly comes off as a smoky creature. When I read aloud a piece with a supervillain, I gave the villain a maudlin British accent, which helped him appear even goofier.
In the case of a novel, individual voices are necessary for a novel where every character gets a say. Not just the villains and the heroes, but the anti-heroes, the helpers, the thugs, and minor people who steal the show. Democracy has a price, but it gives our novel strength.