Random Inferno point

In Inferno there's an odd moment when we and Mark hear a woman's laugh on the soundtrack. He asks Elise if she heard it, but she did not. Our perspective and Mark's are thus shared, perhaps creating suspicion over Elise's identity and function within the narrative. Did she really not hear the sound or is she lying and, if so, why?

Earlier, however, on the discovery of Sara's body, there's a scream as it rips through the curtain, half in the non-diegetic soundtrack, half not, the film being one which has a complex sound design that, as is often the case in Argento (cf. the 'turn it down' moment in Tenebrae, Edda Dell'Orso's vocalism in the gallery sequence in The Bird with the Crystal Plumage) blurs and confuses such boundaries. It is not Sara, as she is already dead. So who is it, why does Mark not seem to hear it and what, if anything, is the reason for the difference between the position and response to these screams?