Using your Chromecast built-in audio device.
To start using Chromecast built-in with your audio device, connect your audio device to a Wi-Fi network using Music Center / SongPal, then download Chromecast-enabled apps to your mobile device.
How to connect your audio device
See here for details on how to connect a device.
Download Chromecast-enabled apps and enjoy music
Download Chromecast-enabled apps from Google Play or the App store to your mobile device.
Start a Chromecast-enabled app and play back music.
Tap (Cast button) and select the audio device. The Cast button will turn , letting you know that the audio device is connected.
If you play back in the group (Wireless Multi-room), create a new group using the "Google Home" app. For details on creating a new group, see here.
Music will be heard from the audio device.
For details on how to use Chromecast built-in, follow the links below.