Learn Finnish: Museossa kuultiin hälytys. - An alarm was heard in the museum.

Kuulla (to hear) is a type 3 verb. To form the passive past for type 3 verbs, we add -tiin to the consonant stem (kuul-). This gives us kuultiin (was heard).

Why is the object hälytys in its basic form, rather than taking an object ending like -n?

In passive sentences, a complete whole object doesn't take the usual -n ending (the accusative case). It stays in the basic nominative form. So the word for the alarm is simply hälytys.

Can this sentence also mean 'We heard an alarm'?

Yes, but usually you would add the pronoun me (we) to make that clear: Me kuultiin hälytys. Without me, it is most likely the true passive 'An alarm was heard'. However, the verb form kuultiin is exactly the same one used for the spoken 'we heard'.

Why does the sentence start with Museossa?

Passive sentences don't have a specific person doing the action, so they lack a subject. Because of this, it is very common in Finnish to start passive sentences with a location or a time to set the scene. Starting with Museossa (In the museum) naturally anchors the sentence before telling us what happened.