if it's in relation to something they are saying specifically that is triggering you, it could be dissociation. If it's that you get bored and what they aren't saying is interesting enough to hold your attention, that could be a sign of ADD or something of the sort. If you literally can't make a distinction between the sounds coming out of their mouths, it could be an auditory processing disorder or something like that. I would consult a professional about it.