Voice is one of the most important aspects of a story, and it's something that has to be established early on.
I'm going to give you my Little Mermaid analogy.
The first time Ariel is supposed to be shown in the movie is at the concert in the beginning. Sadly, she didn't make it, but the important thing is that it shows her strength right from the beginning. It's her voice (I know, a little cheesy but it worked for me). She is the best singer of all the sisters...and also a little bit of a slacker in the merworld.
Think about the one thing that Eric remembers Ariel by. The one thing that connects him to her and defines who she is in his eyes. It's her voice. He heard her sing and that's how he recognizes her.
So, a character's voice is what drives them in the story. It's what makes them do what they do. When Ariel is signing a contract with Ursula, Ursula wants to take what's most precious to Ariel. Ariel uses her voice to get what she wants most!
Sadly, without it, Eric doesn't know her. He thinks she's familiar, but he's not positive.
If your character is lacking voice...they WON'T be recognizable. It defines who they are and is the reason they act and react in a story.