My old cat Fluffy who died severaly years ago use to open his mouth to meow, but no sound came out. Maybe is was just so faint that I couldnt hear it. He also hissed but made no sound! He opened his mouth and made all the actions just no hiss! We use to say he was, 'smiling'! He was around his mom and dad, brothers and sisters when we got him so he learned but he just couldnt make the sound. Maybe there was somehting wrong with his vocal cords! He didnt mind though he kept up 'smiling' and pretend meowing till he died Ohhh I miss my Fluffy boy.