For the record I use my app Nyckelharpa, specifically the Talker module to handle these messages, but just tried this using RM

Warning: I use a modified (now outdated) FKBC version that I ported, released, and used on ST that also works on HE. When using the current HE version I believe the message looks like the following, / are gone (added by HE app's code), and speak commands are not used.
Alarm system is arming in 30 seconds, please exit the facility. {break time="1s"} 25 seconds{break time="2800ms"}20 seconds{break time="2800ms"}15 seconds{break time="2800ms"}10 seconds{break time="2800ms"}5{break time="700ms"}4{break time="700ms"}3{break time="700ms"}2{break time="700ms"}1
In a worst case scenario it will simply speak all the control information if it's miscoded. You may also need to adjust the delay break times.
Important. Since arming may be canceled you will need a rule issuing message !System Disarmed. An ! preceding the message clears the message queue and stops the currently speaking message, prior to speaking the message.
The speak command code is around line 305 in the HE FKBC
The documentation for the ST FBKC also has links for a few suggested sound files