Why do silencers make bullets weaker and not stronger

Sound is energy and the bullet is using energy to leave it's chamber, so why does severely decreasing the sound not give the bullet more energy/ speed instead of decreasing it's speed/ energy?

Well, the first part of the answer is that the pressure absorbed by the silencer is NOT applied to the bullet, so how could it increase the bullet in any case. And while I don't know the exact physics of a silencer, if it does slow a bullet, I would look at the fact the silencer is added to the end of the barrel and still surrounds the bullet for several inches - without the silencer there the bullet is pushed by the shock wave of the expanding gas that makes the sharp noise, certainly not slowing it and maybe speeding it up, but if the gas is slowed abruptly it might be like sticking your hand out a car window at speed - lots of drag.
In fact, a clue to this thinking will be found here http://www.varmintal.com/aeste.htm, (search down to N-133 Pressure Curves) where the exit pressure on a normal barrel actually starts to drop (less pressure) before the bullet clears the barrel. So would have to find similar curves with silencer - which I don't care about looking for.

as the bullet enters the silencer chamber it encounters air resistance. The chamber doesn't allow the air to easily escape so the air is compressed creating more resistance as the bullet travels through. The desired effect of muffling the sound is achieved at the expense of slowing the velocity of the bullet and generating heat.

silencers compress the sound decreasing the velocity of the bullet think of it like pushing air into the barrel to slow it down and keep vibrations in the air to a minimum

I don't think silencers change the effective hitting power of a bullet. My understanding is that they baffle the sound after the bullet has passed.