들리다 is a sound reaching your ears by itself, where 듣다 is you listening. 어디선가 들리는 목소리, 'a voice from somewhere', is a stock caption for something heard off-camera, and it appears four times here. With 뭔가 it means catching something without catching all of it: 다 이해는 못하지만 뭔가 들려요, said of overhearing a language you half know.