As metaphors go, it’s hard to think of one that has such broad utility in a variety of situations. It delicately straddles the literal, descriptive and comparative.
For singers, it’s an obvious (and practical) problem; how to convey the song in a manner that’s not just tuneful, but personal. In some extreme cases, the latter actually subsumes the former. The most compelling example is probably Willie Nelson, a guy whose voice on a strictly technical level (range, timbre and power) suggests a guy ill-suited as a professional singer. But with considerable critical acclaim and millions of record sales, his legion of devoted fans obviously disagrees.
In writing, the quest to speak to your audience from somewhere “inside” is largely about engaging a personal style that is consistent with your habitual manner of expressing yourself verbally. That’s the only way you can make the writing compelling and authentic.
As a recreational guitar player, I’ve noticed more and more the notion of “voice” in your playing. There’s an anecdote about some of the modal, scalar approaches to melody and soloing that prompted one wag to comment: “Their own mothers wouldn’t recognize them on the radio”.
I struggled with the notion of finding my own voice as a lawyer. Early on, I thought it might be helpful to emulate the style and approach of a lawyer whose work you admired or respected. I found that it was almost impossible to stay “in character” for any length of time if you were effectively (or not) doing an impersonation. It took many years to find my own specific, natural and sincere approach to the craft; in all facets, a case of finding my voice.
I’ve watched a young lawyer at work in my area recently. She’s struggling mightily. Most of the time, she’s busy complaining that the judges don’t take her “seriously”, that they’re “unfair” or that they just don’t give her a break. I don’t have the heart to tell her that all this is happening mostly because she doesn’t yet know who she “is”. Only after she “finds her voice” will anyone take her seriously.