Welcome to the forums!
There has been a good deal of talk on the forums in regards to volume of the speaker when arming the HMS system.
A couple of quick thoughts… If you keep the HMS system in a cabinet, as you know, will reduce the sound of the siren if triggered. You may want to turn the delay time for the "home’ mode to zero seconds so that it’ll arm instantly without having to hear the dinging sounds… (As there is no way to control the sound volume separately.)
As far as extra external siren… That is indeed a good idea an has been proposed on the wish list… But unfortunately at this time, there isn’t an external speaker option. Although it also has been proposed that the siren from the V3 cameras act as additional siren speakers…(although currently, there is no way to activate that other than manually setting them off…(Even if you were to set a shortcut, you will still need to manually activate and turn off the camera siren…)
All good thing coming hopefully in future updates…