If you hear only a static or crackling sound when the siren is activated, unfortunately your siren is probably just a dumb speaker. It requires a siren driver module to modulate the siren sound. Most of the time the siren driver module is built-in to the old alarm panel, so you can’t use it with Konnected.
In this case you have a few options:
The easiest and cheapest option is to replace the siren with a newer, inexpensive siren. The Honeywell WAVE-2 is my favorite because of its incredible value.
You also can buy a siren driver module. In this case, just connect your siren to the driver, and put the relay between the siren driver and 12V power.