We went out on safari again in the evening, and at night fell asleep listening to the hippos.
Literal
Evening [at-に] again safari [to-に] going-out, night [topic-は] hippo [possessive-の] voice [object-を] hearing [while-ながら] sleep [to-に] attached.
眠りについた uses the idiom 眠りにつく ('to fall asleep,' literally 'attach to sleep'). ~ながら ('while') connects hearing the hippos with falling asleep — a lovely sensory detail. サファリ is the loan 'safari.' カバ ('hippo') is the standard Japanese abbreviation of カバ (from Dutch/Portuguese). The sentence paints a vivid travel scene.