。
The siren announced the sudden emergency.
Literal
Siren [subject-が] sudden-change [object-を] informed [polite-past-ました].
急変 ('sudden change, sudden turn for the worse') is a formal/medical word often used for patient deterioration or urgent situations. 知らせる ('to inform, notify,' from the causative of 知る 'to know') is formal for 'announce.' The personification of the siren as the subject is common in Japanese — objects and forces can 'inform' or 'tell' without grammatical awkwardness.