Go to Settings > Sounds & vibration > Smart volume > Music and video playback to enable this feature.
When “Notify me when in operation” is turned on, a toast notification will appear in the playback of the first media file (music or video). The toast notification will not appear in any following media playbacks if the screen is not turned off. This is to avoid unnecessary interruptions when multiple media files are consecutively played back.
Note: This knowledge is currently only applicable for ZenFone 6. Please stay tuned for further updates on ASUS Official Website.