Sometime in the last day or two, the Pixel 6 and Pixel 6 Pro picked up the Adaptive Sound feature that debuted last year on the Pixel 5 and 4a 5G as part of the December 2020 Feature Drop update. The feature uses the phone's microphone as a feedback mechanism to dynamically improve speaker performance based on your environment, which was particularly handy for the Pixel 5 and its mediocre screen-vibrating earpiece speaker. Now the Adaptive Sound can help improve the middling stereo speakers on the Pixel 6 and 6 Pro as well.
If you've never used the feature or haven't owned a Pixel, it's a subtle one, adjusting your phone's equalizer based on the acoustics reported by the microphone as it plays content. In essence, it's able to figure out how to adjust speaker performance to sound better in challenging environments. Of course, the feature needs to record audio to work, so it is optional, but all processing happens on-device, and recordings aren't saved for long periods.
Though the Pixel 6 and 6 Pro technically have stereo sound via the earpiece and bottom-firing speaker, audio performance from both phones wasn't particularly good — even the front-facing speakers on the Pixel 3 series sound better to our ears. It isn't clear if this will fix the issue with imbalanced audio the 6 series' speakers suffer, but it should otherwise improve audio quality in certain environments if you listen to music, podcasts, or other content without headphones.
Android Police's Max Weinbach first noticed the feature on his phones last night, and the rollout was spotted by more people this morning. We separately confirmed that the feature is genuinely new, as it appeared on a Pixel 6 Pro that had been powered off for the last three days within three minutes of being turned back on.
The feature appeared within three minutes of a Pixel 6 Pro being turned on without an app update.
Adaptive Sound is almost certainly controlled by the Android System Intelligence app (the new name for Device Personalization Services as of Android 12) and seems to have been triggered server-side without a corresponding update within the last day or two. It isn't clear why it wasn't available at launch for the two phones and was only recently enabled.
Pixel 6 and 6 Pro owners can take advantage of improved speaker performance by enabling the feature at their leisure in Settings -> Sound & vibration -> Adaptive Sound.