When things are being cast to a Chromecast or Wi-Fi speaker, perhaps not from your phone, a notification can appear which gives the original device control over playback, volume, and everything else. While this is useful, it can still be inconvenient if you need to figure out the original cast device just to complete a simple task.
Now in Google Play Services version 10, actively playing Chromecasts on your network will show a notification which gives you control from any of your devices according to a report from Android Police.
You can start downloading the Google Play Services update from APK Mirror if you don’t want to wait for it to roll out. Right now this is not available on Android TV, but it wouldn’t be hard to add it to a future update to its cast receiver.