-After the sewer, go to the apartment 5F. There will be a room with a large hole. There is mattress near the hole. To the right of it is a wall that that has a light blue color section. Use a pipe to break the wall and get the silencer.
-Complete the game to unlock the Flamethrower, Unlmited Sub Machine Gune, and the Beam Saber.
-To unlock these two extra-weapons without having to play a new game from the start, save your game in the tarot-puzzle room just before the last Boss. When you finish the game, you will unlock one of the weapons (Flamethrower or Beam Saber). Save your results. After this, load that previous saved game and complete it again. You will now unlock the other weapon.
-Complete the game and save your results. You should have at least one special weapon unlocked (Beam Saber, Unlimited Sub Machine Gun, or Flamethrower). Then, load your saved game again. Completing the game the second time with the same save will unlock the Flamethrower (if you do not have it). Finish the game the third time, but kill the last Boss, Claudia, with only melee weapons such as the Maul or the Katana. After that, you will unlock the Unlimited Sub Machine Gun. Note: If you do not have the Beam Saber, then in the third time through kill the final Boss, Claudia, with long range weapons such as the shotgun or the sub machine gun.