I don't think KM has any method to send audio to my HomePods speakers (which probably isn't KM's fault, since I assume Apple doesn't have an API on macOS to support that.) In fact, "macOS 26 (beta)" also does not seem to support the Intercom action in Shortcuts, from what I read about it. Can anyone with the macOS 26 Beta confirm this? If it gets implemented, we should be able to get KM to do this, indirectly through macOS Shortcuts.
However I have managed to accomplish a "daily audio message sent as an intercom message to all my HomePods" by creating an iOS automation that is triggered at a specific time of day each day and uses the Intercom action in Shortcuts to remind me to do something. I would really hope that someday KM could support this, but I know KM is limited by what macOS allows via public APIs.