Both the sirens described by @rwclements228 do not have TTS engines. They are essentially players of WAV/MP3 files. So, they will only work for pre-recorded static messages.
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Unfortunately, to use TTS there is no good way to do it locally.
I have Alexa devices in multiple rooms in the house. On critical alerts (intruder, water leak, etc.) I use both the local Aeotec device as well as Alexa TTS. If the Internet is operational, I would get multiple alerts. However, if an intruder were to cut my Internet cable coming into the house, I would still have the local alert. One thing I have learned is that the local alert occurs a few seconds faster than the Alexa TTS alert. It takes time for Amazon to receive the text, convert it to audio, and send it back to the Alexa devices. Thus, in critical situations, local alerts are better.
As I remember, mycroft (aka picroft) has a fall back voice for when the desired voice/system is not available. This fallback is slightly less crappy than the Linux text to voice, espeak: $ espeak 'hello there' As I remember, OSX has the same command or something similar.
