What are the quantities that remain unchanged after reflection ?

Wavelength, frequency and speed of the sound waves remain the same.
These quantities remain same because reflection is bouncing of a wave in to the same medium. As because the medium remains same velocity remains same. Now velocity of a sound wave is related to its frequency and wavelength by the relation V = νλ. Now as V is constant product of frequency and wavelength too remains constant. Then frequency of sound depends on the time period of the oscillator that generates the sound thus frequency does not change. As frequency and velocity does not change, wavelength too remains constant