Nest presence detection by the Protect devices is only used by the thermostat via 802.15.4 LR-WPAN
on a Zigbee chip. It is recorded in the nest cloud, but only uploaded once per day.
Announcements are not possible with the Nest Protect. Those messages you do hear are fixed in the firmware. However, if you want to make announcements in your home, you can use Google Assistant Relay to announce on Google Home devices via TTS. The announcements will be made via the default voice you have selected in the Google Assistant app. You can also control any device or trigger any routine setup in Google Assistant, with or without response. There is also some good progress in using Alexa devices for TTS and to trigger routines and you can expect to see official capabilities to do this via the Alexa Skill in the near future.