Samsung's Bixby Voice early access program seems to have begun hitting U.S. Galaxy S8 and Galaxy S8 Plus devices, though to be included as one of these individuals you would have had to sign up to be a part of the program to get an early version of the digital assistant feature that many thought would be available on the Galaxy S8 and Galaxy S8 Plus at launch. It's reported that it may take the next few days before the majority of users who signed up to be a part of the early access program receive the update that enables the use of the new function, so if you signed up to be a part of the program you'll want to watch your device alerts closely for the next few days to see if you end up getting early access to Bixby Voice.
On top of checking to see if Bixby Voice has been enabled on your device, you will also likely need to ensure that you have the most recent update for all relevant Samsung Apps that can interact with Bixby. If you're unsure whether or not you have the most updated versions of the Samsung apps, enter the Galaxy Apps Store and check to see if any updates are available for you to install, which should get you to where you need to be to use Bixby Voice with the apps that support it.
Samsung only just recently announced that it would be starting an early access program for the Bixby Voice feature as early as last week, so it may have been expected that access to the feature would start sometime in the future, but at the time Samsung did not officially confirm any time frames. Now that the early access program seems to have started though, Samsung can begin working towards getting Bixby Voice out to all Galaxy S8 and Galaxy S8 Plus users, no matter what region they live in and regardless of which model of the device that they have. If you're not part of the Bixby Voice early access program, you can still sign up for a chance to receive access to the feature,