Yes, a flash suppressor does make a gun louder. It does this by trapping the gas and hot particles created by the explosion of the bullet. This gas and hot particles create a pressure wave that is released when the suppressor is opened. This release of pressure wave creates noise.
A muzzle brake does not make a gun louder. It redirects the gases that come out of the muzzle so that they push against the gun less. This reduces recoil and makes the gun more stable. Muzzle brakes are often used on sniper rifles and competition pistols.
A muzzle brake will make a gun louder if it does not have a good seal. When the brakes are first installed, they need to be checked for this. A good seal will prevent gases from escaping and will keep the noise down.