During the scene where Luke and Leia are trapped on the catwalk Leia fires the stormtroopers rifle several of the shots sound like gun fire not blaster fire. They did use blanks to create a muzzle flash so the effects people new when to put in the blaster beams, so it appears they forgot to replace them with the correct sound effects.
The reason it only shows up in the special edition is that lucasfilm went back to the "original" sound recordings (the ones used in the first release of the film (the one without Episode 4 a new hope at the beginning)and duplicated the same mistake it made back then. The sound effect is the one used for a 44 magnum in those clint eastwood films.....Dirty Carrie?