I’ve moved your post over to #support so we can help get this taken care of for you.
So we can better assist you, please provide a brief description of your current setup using this link as a guide.
Make sure to describe your network configuration/topology, including any networking hardware currently in use, so we can have a clear understanding of how your devices are communicating.
Is this happening with all types of content on Big Sur? Is this just when playing to the internal speakers?