。
I heard a little girl loudly crying for help.
Literal
Young girl [subject-が] loud-voice [with-で] help [object-を] seek [nominalizer-の] [subject-が] was-audible.
~のが聞こえる ('hearing [that X was happening]') — の nominalizes a clause, が marks the nominalized clause as subject of 聞こえる (spontaneous perception). 助けを求める ('ask for help / seek rescue') is a fixed collocation. 大声で marks the manner.