Hello Lmb45,
We see that you're losing sound when trying to complete a workout with Fitness+. We would like to see how we can help.
Let's start by having you force close, or remove the Fitness app from the Dock, restart your Apple Watch and test to see if the issue remains. The article will guide you through those steps, if needed.
Open apps on Apple Watch
How to restart your Apple Watch
If the issue remains, get back to us with some more details. Does this happen with every video, or just certain ones? Do you have an issue with the audio when using other apps? Are you connected to a wireless or cellular connection issue when this happens?
We look forward to hearing back from you.
Cheers!