I hope you solved your problem, as i have exactly the same one.
A few month, year ago, i had only one switch, and now, UI7, i have the two switch "Arm/disarmed" and "On/Off".
But i'm more lucky, as i can set the siren from luup code, to switch it on or not (i mean sound).
I still don't understand the functionnality for the Siren to be armed or not...
Anyway, for me it works for what i want(but i have the same problem from the Android app, when the Siren is on (sounding), i can't swith it off, as i only have the Arm/Disarmed switch displayed, and nothing happens when i swap it), but i have an issue, and here i go:
I use to have the Siren appear in the Panel Mode, on the second section "if one of those sensors are armed and tripped, what happens...".
But now, the Siren disappeared.
So i had to create a scene to turn on the Siren when one of the sensors are tripped.
The thing is that the scene does not wait for the "30" seconds i've set in the Panel Mode, to wait to do something after sensor triggered...
I use to have a code luup, dealing with all those delays (like we all did i suppose), but i decided to remove it and use the builtin code linked to the Panel Mode.
My first investigation is that, as my Siren does not appears anymore there, i need to understand why.
Could you please tel me:
If your Siren appears in the Panel Mode.
If yes, can you give me its "Category_num"? (for me 3 for now, and the Siren is showed in the Devices Panel as a light and plug)
Thanks a lot for your reply!