My alarm siren failed earlier this year and resulted in the alarm randomly going. Oddly, it did actually make a noise, so not sure exactly what was failing. I first noticed it when returning to the car and seeing the hazards going off. Then one day it started going off at home and eventually I couldn't lock the car at all. I removed the alarm siren and didn't replace it as it would have cost £200 or something stupid and alarms are pointless. The car still has an alarm, it just won't make a noise (can be tested and works like Snoopy said).
The metal cage is riveted in, annoyingly. So needs to be drilled out and then I suppose if you want to replace it you can rivet or bolt it back, but not sure about that.