Audio problem: There is a scene where Eric is following the crow through a dark room. The camera trails off to show the crow's shadow and there is a sound as of wings fluttering yet the shadow is perfectly still. (00:10:10)
Audio problem: When Albrecht asks to Eric if he is going to vanish into thin air again, the shot changes and Albrecht still is moving his mouth but now he is voiceless. (00:46:30)
Audio problem: When Shelly is being wheeled away in the beginning, she makes a strained gasping sound right before they put the oxygen mask back on her. But her gasp doesn't match her mouth movements. In fact, it's impossible to make the sound she made with the mouth movements she made. Try it yourself. It almost looks like she's saying "F**k" or something similar, and they just dubbed it over with a gasp. Additionally, the gasp is far too loud and clear given how softly she's speaking. Bad looping.
Answer: There is no explaination given in the films, they are just brought back when the crow feels it is the right time to avenge their murder.