Have you ever accidentally started the signup process for a website you already use, or otherwise pressed a button you didn't intend on pressing simply because it looked like the right one, or looked like a button that would perform the intended behavior?
Increasingly, interfaces (particularly on the web) are shifting from giving equal visual and tactile attention to all actionable buttons to developing their own voice that reflects what the interface suggests you should do, not necessarily what you want to do.
Here's an example:
This is kind of perplexing - after all, aren't returning users just as important as new ones? Why does "Sign in" just look like a text link and not a button?
The company stresses in their FAQ that to get the most out of Dropbox, you should be using the desktop client. So while Dropbox has over 200 million users, it wants those users to be using the desktop client, while those visiting the site probably haven't signed up, or probably won't log in to the website that often.
TripIt is another site that differentiates buttons based on whether you're a new or returning customer. If you didn't read the text of the giant orange button, you'd be forgiven for entering your username and password and hitting enter. But the heavily de-emphasized button in the upper right of the signup interface is what you actually want.
In this case, it's a little more perplexing. Yes, TripIt has mobile apps and can automatically scan your email for trip information, but even TripIt can't say with a straight face that the apps have more utility than the website. If the service gets something wrong, or otherwise needs action, you essentially have to use the website. So I'm not clear on the logic behind this interface.
Another quick example is Pinterest. The area highlighted here in orange represents the space dedicated to getting more people to sign up, while the rest of the area shown is dedicated to welcoming back current users.
One more example is Android's first-run process for adding a new Google account. This is kind of a double whammy. First, instances of free-floating text-only buttons in Android's stock apps is positively scant, so this isn't exactly a familiar element. Second, the "Continue" button, which illuminates once you've made a choice is on the right, the same side every forward-moving button is on in the rest of the interface. Third, it even has a Wallet icon next to it! Oh, and the copy up top. "Enter your billing information."
It's important to note that entering this info is essential for buying apps or other content with a given account, but this sort of strong suggestion is a little distant from Google's usual friendly voice. But it's a great example of seeing what the interface (or perhaps more appropriately the entity behind it) wants you to do, and communicating that silently through interface design. And this is what I call "software voice." A voice embedded in the software, communicating by proxy an entity's suggestion for action.
In these examples the voices are largely motivated by the interests of the companies in question, but I think it is possible for software voice to be friendly, too, or at least not so forceful.