If you are having a hard time hearing the siren, try to remove the cover (no screws) and check the following.
A customer reported the following:
I took it apart and that component was lifted. Almost vertically. I put it back down and it works now!
Resetting the wireless Siren:
https://osigodirect.kb.help/how-to-reset-and-re-add-the-gen-2-wireless-siren/