I think brand voices are like people's voices: a collection of qualities, rooted in values and experiences, that change with context.
Say your brand values X (ex. clarity, optimism, thoroughness). To begin building a brand voice, we ask: what are writing strategies and content topics to help you write with X?
Then we create a guardrail: You are X, yet you are also Y (ex. heartfelt, precise, breezy). What are writing strategies to help you write with Y?
Then we do that for all your values. This gives you a unique brand voice and a sandbox full of ways to bring it to life.
It allows your brand voice to attain consistency while remaining flexible to the roles and skills of the people who use it, writers and non-writers alike.