Given the lack of script-friendliness of the current incarnation of the System Preferences app, perhaps a more fruitful approach would be to use this answer on Ask Different to change the setting directly.
So to turn on time announcements, you would use both of the following:
do shell script "defaults write com.apple.speech.synthesis.general.prefs TimeAnnouncementPrefs -dict-add TimeAnnouncementsEnabled -bool YES "
do shell script "open /System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/SpeechSynthesis.framework/Versions/A/SpeechSynthesisServer.app"
And to turn off announcements, you would use just
do shell script "defaults write com.apple.speech.synthesis.general.prefs TimeAnnouncementPrefs -dict-add TimeAnnouncementsEnabled -bool NO "