I have heard the effects of missing syllables in speach and a lot depends on context because something that is missing in a familiar passage (say speach) might not be where the stress of recognition lies, or it might sound like a glottal stop and stand-out. In terms of music - with a familiar genre &/or a tune it will be more obvious but if it was Karl-Heinz Stockhousen (in a John Cage) how would you know? And that is before we consider the listener, as has been mooted.