sound travels through a thread in a longitudinal wave .........the energy pushes the 1st particle and the 1st particle exerts force on the 2nd particle to move on by creating a region of low pressure and high pressure which we know as rarefaction and compression.and that's how a sound energy travels through a thread .

i hope it helps................  
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thank u ..pahiroy..
ur welcome manand
Actually what happens is.....sound waves via vibrations pass through the threads's end A to end B.!! as the sound vibrations strikes a human ear at the end B..those are converted as messages for our brain and we get to know what the person at end A is reciting .!! sound vibrations transmits in the thread so hence , we get to hear sound in end B when a person speak something at end A ....
Hope it helps.!