There are a few things to think about here.
First what smarthings hub do you have and when did you get it. If you just picked it up you may want to return it and look at the ADT Smartthigs panel. That is a alarm systems that also has smartthings built into it. The two environments are silo’d from each other, but some information is passed.
If that isn’t an option you probably need to use something like Smart Home Monitor. That is a Smartapp that will setup a kind of home security system. When you setup SHM you get a cloud based alarm system that will monitor door/window sensor, Motion sensors, smoke sensor, CO Sensor. I believe you can tell it what siren to use. As HBR indicated once the Arlo cameras are added all of your cameras and the built in siren appear in Smartthings. Since the siren is in the base that is the device for it. If you don’t rename it that is the device you want to select for it.
2 things to think about either way you do this. Frist The Arlo Siren is a cloud based device. That means if the smartthings cloud or the arlo cloud is having issues there is a chance it will not go off. Along with that the SHM application somewhat functions in the cloud. This means there is a chance for it to not work either.
My suggestion if you really want security to be a priority is to get the ADT Smartthings panel because it’s security functionality is all local and doesn’t depend on Smartthings to work. It will include a built in siren. It isn’t as loud as the Arlo though. If you choose not to do that, then at least get a dedicated siren that connects directly to the hub.
If you decide to get the ADT Smartthigns panel look at the ADT Tools 2 Smarttapp. That will allow you to use the siren with your alarm. If you go with the SHM i believe it will allow you to select it as a siren device when you go through the setup.