When you start playing music on your smart display, you’ll see a Cast symbol on the bottom left showing what device is also streaming the song. This will work with most audio apps, including Spotify, Pandora and YouTube Music. Tap the badge and you’ll be taken to a separate panel with all your home’s connected speakers (or Cast-compatible output devices including Chromecasts) on a list. You can tap the devices that you want to have playing the music and use an onscreen slider to tweak how loud each one is. At the top of the list is a master control for volume that allows you to quickly soften the music when it’s too loud, for example.
During a demo for Engadget, Chan played Taylor Swift’s Cardigan on a Nest Hub Max, then pulled up the list of other Assistant-enabled speakers available. He tapped two other devices to add them to the playback, and dragged their sliders to adjust volume. When he pulled the master volume slider to the left to soften it, the selected speakers’ volume sliders also moved correspondingly. You can remove devices from the grouped playback by tapping the circle to its left, like you would when unchecking items on a list.
Since this control interface is visual, it obviously won’t be available on speakers. But if you don’t have a smart display in your home, you’ll still be able to use this control panel on the Home app on your Android or iOS device when it rolls out on those platforms later this year.