When an objects vibrates back and forth , it sets the air particles around it in motion . These particles further vibrates its surrounding particles and this process continues till these vibrations reaches our ear and become a sound .. 
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As we know that compressions are high density area and rarefactions are low density area.
In order to right side of the vibrating body!!!
When an object vibrates right it compresses the area present near it which creates a high pressure area and compression is formed.
When it vibrates to the left the area where the body was,becomes wakened and the surrounding are spreads there and a low pressure areais formed this creates a rarefaction.
This process continues till the body vibrates too and fro.