Your Apple Watch will be able to give a Siri voice report on your workout progress with watchOS 81
You can listen to the audible announcement of your workout via AirPods or other wireless earbuds
If you want to use this voice feedback option, you will first need to enable it. You can do this from the Settings app on the Watch or through the Watch app on your iPhone, from Settings > Workout. There, the option is available with a toggle to enable/disable.
When activated, a voice announcement will give you your workout progress. For example, you will hear it when crossing into the next mile during a run. Additionally, it will be able to report when you have reached the halfway point of a timed session.
With this new feature, you won't have to always look at the screen to get an update of your workout progress, as you'll hear the occasional checkpoints via your headphones.
The Voice Feedback seems to be synced with the moments your Workout app on Apple Watch would buzz with a status alert. Siri will be announcing something in the likes of "Mile two, pace 6 minutes 30 seconds". Siri will also alert you when you close Activity Rings during a workout or when you meet your workout goal.
If you're listening to music or a podcast during your workout session, the Voice Feedback will not interrupt it, but announce its report over it. Keep in mind you will have to have your earbuds paired to the Watch. If they're paired to your iPhone, the Voice Feedback will not play.
Currently, watchOS 8 is a beta version, and its official release to the public will be in the fall.