Thanks for reaching out. What's the make and model of the Bluetooth Speaker you're using? Is this only happening when streaming videos from Disney+ app or across all apps? You may try to disconnect the connection of your Bluetooth speaker and Chromecast then reboot the devices. After that, please try to test again.
Also, if this issue is isolated when using the Disney+ app and it's working when the Bluetooth speaker is disconnected, please reach out with Disney+ support for further help.