Noise app
How do I get the Noise app onto my iPhone 13 for my 1st generation SE watch?
Apple Watch SE, watchOS 9
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Make sure you have not turned it off - see this article
Use the Noise app on your Apple Watch - Apple Support
Watch app, My Watch, Noise
If the app does not appear in the list of apps, or you ave deleted it see here
Delete apps from your Apple Watch - Apple Support