Here is an example of how to stream a MP3 to your Google Home:
require_once("Chromecast.php");
// Use the local IP of your Google Home, and the port is 8009
$cc = new Chromecast("192.168.0.31","8009");
// you can change the volume to 100%
$cc->DMP->setVolume(1);
// Play the MP3 file
$cc->DMP->play("https://file-examples.com/storage/fe8907cad7625c93f9981b0/2017/11/file_example_MP3_700KB.mp3", "BUFFERED", "audio/mp3", true, 0);
Here is an example of how to stream a MP3 to your Google Home: