A writer, I believe, should always read his work aloud.
Trust me, that's not really as self-evident as it sounds, having now been through the process from start to--well, nearly finish. Writing is, after all, a solitary and, except for the motivational music that many writers utilize in the background, a quiet pursuit. Yes, some good people write screen plays and stage plays that are meant to be read aloud, but I'm not limiting my suggestion to those folks. All writers should read their work aloud before passing it off as complete.
Part of the reason I perform that ritual is, honestly, anchored in narcissism. It feels good to hear words that I craftily placed in a specific order uttered out loud as though they matter, even when the voice doing the uttering is my own. It's a silly thing, but why else am I writing? It's not for the money. Yes, some authors--a very few--make serious bank on their writing skills. I'm hoping to be one of them. Still, hoping is one thing, but expecting that type of success would just be silly. My editor and others have suggested that the book is, or at least will be, very salable once I'm done with revisions, and for that I'm glad, but I'm also only reasonably looking at a $6,000 - $10,000 payday expectation. I know, that sounds like a large amount. However, if you deduct what I've paid for professional editing, as well as the costs I'll end up sinking in manuscript transportation, you'll end up with a significantly smaller sum. Then, if you take the amount I make on a rough hourly basis in my day job, and multiply it by the number of hours I've invested in this project, you'll see why even the higher number represents an insultingly small compensation. Ah, well. As I said, it's not for the money.
There are a couple more practical reasons for reading your work aloud, of course. One is that you read differently when your mouth is trying to form the words at the same time. I don't know why. I presume that somewhere in the psychology or education literature there's a study done on what happens when you read mentally, to yourself, versus reading to speak the words, but I don't care enough to go find it. Whatever the literature says, I'm much more likely to find errors in the text when I'm reading aloud. I think it has something to do with the brain's ability to compensate for syntax when I'm blazing across the printed word. Last night, for example, I excitedly printed off the nine perfect pages of the newest chapter ("Here, Kitty Kitty!") and ran in to the other room where my family was, looking forward to the joy not only of listening to my own work but also of having them heap me with praise--or the other stuff, depending on which it deserved. I started reading, and didn't get half of the first page out before I came across an error I hadn't seen. I'd used a singular noun where a plural was needed. I'd gone over it several times at the keyboard and hadn't caught the error. One verbal read through, though, and gotcha!
The other reason to read aloud, I think, is because a well-constructed bit of prose sounds--well, good. Well-constructed, as it were. It's especially true for combat sequences. When you've written a good battle scene, you can pass your eyes over it dozens of times without really hearing the rhythm of the linguistic dance that the words take on. When you read it aloud, though, it's clear when you have a winner. The piece becomes more than just a collection of words. The sentences and paragraphs seem to leap into life.
And it only happens when you read it aloud, I've found.