Some observations about developing a writing voice, gathered from years of paying too much attention to how sentences work:
Voice isn't something you build. It's something that's already there, buried under all the habits you picked up trying to sound like a proper writer. The work is removal.
You can hear Kurt Vonnegut in a single sentence. That flat, midwestern delivery. The way he'd say something devastating and then follow it with "So it goes." He didn't develop that voice. He just stopped disguising it.
Most people who are "searching for their voice" are actually searching for permission.
I've noticed that writers tend to find their voice faster in emails and text messages than in manuscripts. There's something about the absence of an imagined audience that strips away performance. The trick, if there is one, is to write your essays with the same looseness you'd write a note to a friend who already gets you, and then tighten from there rather than the other way around.
Read Raymond Carver's early drafts, before Gordon Lish edited them down to bone. Then read the published versions. It's genuinely hard to say which voice is "Carver's." The stripped versions are the ones we remember, but the longer drafts feel warmer, more human. I go back and forth on whether Lish found Carver's voice or replaced it with something better. I honestly don't know the answer.
Voice is rhythm before it's content.
Joan Didion wrote in "Why I Write" that she was "not a thinker" but someone who needed images, pictures, to find out what she meant. Her voice came from that limitation. She wrote around the edges of things because she couldn't get at them directly, and that circling became the most recognizable style in American nonfiction.
One way to test if you're developing a voice: read a paragraph you wrote six months ago. If it sounds like you, you're getting somewhere. If it sounds like it could've been written by anyone in your MFA cohort, you're still performing.
People confuse voice with vocabulary. A big vocabulary gives you range but says nothing about who you are on the page. Hemingway proved that with about 200 words.
"If it sounds like writing, I rewrite it." That's Elmore Leonard. And he's describing the entire project of developing a writing voice in eight words.