Google Cast video devices (Android TV, Chromecast) cannot be grouped with Google Cast audio devices (CC Audio, soundbars, Google Home). And devices are either cast senders (phones, tablets) or cast receivers (Android TV, Chromecast). You can't have an Android TV device cast 'outward' to another Google Cast device.
Update: After some tests, it's clear that Android TV can't directly cast audio/video file to Chromecast. But if you have an Android phone, you can cast audio/video files on Android TV to Chromecast device by using the phone as middleman. If you have a ton of media files on a storage, like USB hard drive, connected to Android TV, this might be worth the effort. Otherwise, it's obviously much easier to cast media files from a phone directly.
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