I've started walking with my S23+ and Samsung Health. I'm in the USA so every setting in my phone and all apps are set to miles.
When I started using the app the "audio guide" would announce through my (non-Samsung) ear buds each mile that I walked.
Yesterday I added a Fit 3 . The "audio guide" inexplicably changed to announcing each kilometer, and there doesn't seem to be a setting to change it.
