Yes, it is possible to play audio from a Chromecast device over Google Home Mini. Here are the steps:
First, set up your Chromecast device on the same Wi-Fi network as the Google Home Mini device.
Once your Chromecast device is connected to the Wi-Fi network, you'll need to link it with the Google Home Mini device. To do this, open the Google Home app on your smartphone and select the Home tab. Then, select the “+” icon, followed by “Set Up Device”. Select “Have Something Already Set Up?” and follow the instructions to link your Chromecast device to the Google Home Mini.
Once the Chromecast device is linked to the Google Home Mini, you can now play audio on the Google Home Mini. To do so, simply say, “OK Google, play [name of the audio] from Chromecast”.
For more information on playing audio over Google Home Mini, please check out this article: https://www.digitaltrends.com/home/how-to-cast-audio-to-google-home-mini/.