The Noise app measures the ambient sound levels in your environment using the microphone and duration of exposure. When Apple Watch detects that the decibel level has risen to a point where hearing could be affected, it can notify you with a tap on the wrist.
Note: The Noise app uses the microphone to sample and measure the sound levels in your environment. Your Apple Watch doesn’t record or save any sounds to measure these levels.
Go to Noise > Environmental Sound Measurements, then turn off Measure Sounds.
You can also go to the Apple Watch app on your iPhone, tap My Watch, tap Noise, then turn off Environmental Sound Measurements.