I went in the rules and cant figure out how to speak "silence"???????
Under Automation | Voice | System Miscellaneous, select one of the Miscellaneous. For example Misc #2 is vm240. In the first drop down, select [500ms Silence]. Hint, it is very near the bottom, so scroll up. The other choice is 200ms Silence.
Now go to the Rules and bring up your light flashing rule. Add a "then" | speak | Miscellaneous 2 (vm240).