If the wavelength of a wave in a medium decreases, it's frequency remains unchanged. also...

If the parameters of the medium are constant, the velocity of a sound wave within it remains constant.
$v=\lambda f$
There is no need for physics to explain what will happen to the frequency of an object if v remains constant and the wavelength decreases.
$v=\lambda \downarrow f\uparrow$
To keep the velocity constant, the frequency must rise. And answer is increases.