You didn't mention the model of speakers you have but if they support the Google assistant and Google casting and are available in the Google Home app then look at the speaker group link below for more information.
I hope we were able to answer your question with the info provided by @Cathal_S (Kudos to you!). Just to add, please note that we have very limited support on third party speakers with built-in Google Assistant like Xiaomi. You might need to reach out to them for better assistance.
If you have additional questions, feel free to respond to this thread and we'd be happy to further assist you.
I haven’t heard back, so I'll be locking this thread if we won't hear back from you again in 24 hrs. Feel free to create a new one if you have more questions or have other concerns in the future.