Smart Volume is a feature that adjusts the initial volume of incoming call ringtone and media playback based on the background noise levels. When it's activated, the volume will be louder in a noisy environment (for the media playback, the volume will not be higher than the volume set by the user), and lower in a quieter environment.
To turn this feature on, go to Settings > Sounds & vibration > Smart volume.
In Smart volume settings , you can individually turn it on/off for incoming calls, internet calling apps, music and video playback. For the media playback, you can also turn on/off notification when this feature is in operation.
Note: This knowledge is currently only applicable for ZenFone 6. Please stay tuned for further updates on ASUS Official Website.