In automobile engine exhaust valve opens and high pressure gase enters the chamber this exhaust process creates pressure waves that make noise on exhaust.
Here comes into play role of silencer
Inside silencer there are small tubes which cancels sounds and thereby sound is reduced that originate from engine.
If there are no silencer then there wont be silence :)
Also if there are bikes without them , there will some mechanism that may silence the engine
Find detailed technical description here