what is the frequency that our naked eye can see?
There is as such no particular frequency which our eyes can see, but sometimes the vibrations produced by sound are visible depending on the type of medium.
Sound waves are not always invisible. Sound is simply the ordered vibration of matter. Any time you see a vibration, you are seeing sound. For instance, hit a bell and watch it shake. That shaking motion of the bell is the sound waves traveling through the bell.
Talk quietly to your friend and you indeed cannot see the sound leaving your mouth and entering her ear. This is not because sound is invisible, but is because air is invisible. When you talk, you are sending sound waves into the air and the air itself is what is doing the vibrating.
Also, most sound waves consist of material vibrating too quickly for our eyes to pick out. The sound waves traveling down a plucked guitar string are not invisible. They are just moving so fast that the plucked guitar string looks like a blur to our human eyes.