@bertabcd1234 has described the built-in way to do (i.e. Hubitat uses Amazon Polly to generate a sound file that is played on a speaker locally). As he indicated, this method still requires the hub to have internet access for each new sound file that needs to be generated.
There is a community integration (linked below) put together by @jtmpush18, along with @erktrek, that is fully local. It does require a Raspberry Pi (or equivalent) that is connected to a speakers either directly or via Bluetooth. It can also use Sonos speakers that are on the LAN.
This approach does not require any internet access.
