Thanks for the interesting question. Noise cancellation is applied when World Volume is less than zero. Here are some more detailed notes.
World Volume is the primary controller for Bose Hearphones. Just like the volume setting on the phone controls the loudness of the your music, podcast and phone calls, the World Volume setting controls the loudness of live voices, TV and other environmental sounds. You adjust the World Volume so that it’s just right for you and for each place you go.
The World Volume can be adjusted on the device or using the app. All World Volume settings use Bose Acoustic Noise Cancelling™ technology.
Bass & Treble?
A second adjustment on the Bose Hear app is preferable for some people to optimize live listening. Adjusting the smaller controller on the right of the app screen can makes sounds deeper (more bass) or brighter (more treble). This adjustment does not apply to streamed sounds coming from the smartphone.