When Yoko and I heard the siren stop nearby, we immediately tried to rush outside to see what had happened.
Literal
Yoko [and-と] I [topic-は] siren [subject-が] nearby [at-で] stop [nominalizer-の-を] hear-[and-て], what [subject-が] happened [nominalizer-のか] [quotative-と] right-away fly-out-go [volitional-ようとした] (tried-to-do).
A long multi-clause sentence. 止まるのを聞いて = 'heard [it] stopping.' 飛び出していく is a three-verb compound: 飛ぶ ('fly') + 出す ('put out') + いく ('go'). ~ようとする ('try to do') in past form = 'tried to,' implying the attempt. The quoted embedded question 何が起きたのか marks curiosity.