Sound waves are mechanical waves, that is oscillations of pressure transmitted through a solid, liquid, or gas; sound waves are created by a vibration of an object, which causes vibrations in the air. The vibrating air causes your eardrums to vibrate and you hear a sound. An accordion causes vibrations by pushing air through a valve and over a reed. Pressing a key selects and opens a valve and pumping the accordion blows air over the reed, causing it to vibrate.

The size and mass of the reed and the size of the resonant cavity surrounding the reed control the frequency of the waves. Difference frequencies are different pitches or notes. By pressing a key on the accordion, one can play a note or a chord. The accordion range is from F₃ (with frequency = 174.614 Hz) to A₇ (with frequency = 3520.00 Hz). The speed of pumping controls the amplitude of the wave. The higher the amplitude, the louder the sound.

References:
musicdictionary
waveproperties
soundwaves
piano-key-frequencies
reed-cavity-design
sound