It's because UFOs use magnetic energy to fly not any form of combustion.

Really combustion is a stupid way to do anything when you think about it... you are losing a ton of energy to the heat.

When you hear a jet sonic boom it is the sound waves of the fuel burning collapsing onto itself, when you hear a gun it is the gunpowder exploding. The sound has absolutly nothing to do with it traveling through the air. It's the burning of a fuel at a rapid rate. Think about how loud a jet engine is just idling on the ground, now crank it up to full power and multiply it by 1000 that's where the boom comes from it's all the sound hitting at once.

You do not hear an arrow... you do not hear a gun with a silencer.

I firmly believe that ufos fly using manipulated centrifical/magnetic force.