Buffer's Voice / Tone Guide stands apart from most in that it states up-front that there is no Buffer voice. Each employee is seen as having their own distinct voice, and is free to write as they will — so long as they strike a Buffer-y tone. Which, you could certainly argue, amounts to the same thing as having a voice...
One awesome leader in this space is MailChimp. I'm grateful for their guide, which served as a great example as I set out to write this draft. One interesting lesson is that MailChimp makes a distinction between "Voice" and "Tone." To read more, visit: http://mailchimp.com/about/style-guide/#voice-and-tone and http://voiceandtone.com/
If we follow this language, then, my perception is that Buffer doesn't, at this point, have a "brand voice." We haven't spoken "as" the Buffer Brand. We, as Buffer teammates, each have our own voice. We sign our blog posts, emails, and tweets with our names to reflect that. You never have to worry about not speaking in the "brand voice."
So, in that vein, this is a "tone" guide, and I'm thinking of this as it applies to writing for customers, not the way you necessarily speak internally. :) This is how I can best put into words the goals I have when writing to customers. It is, by no means, final. :) In fact, I think it will do very well if it grows and is shaped by all!
Let's dig in!