I wrote a program allowing any recent Android device to serve as a siren, voice alerts and strobe. I call it LANdroid on the SmartThings hub (see discussion here) and LANnouncer (because Google wouldn’t tolerate “android” in part of a name ) in the Google Play store (don’t worry, it’s free). The web page with everything in one place is here - keybounce.com.
It can use the LAN (as with Sonos) or SMS (so you can receive slightly time-delayed - a few seconds - alerts while nowhere near home.
An Android tablet costs a lot less than a Sonos system.