The sound and volume of your mechanical keyboard rely on several factors like how hard you press, the material of the keyboard's body plate, and the material of the keycap. The sound of the switches themselves come from the key being pushed onto a spring and then released.
Recently, both Gateron and Cherry have released Silent switch products. These switches place a tiny piece of rubber into where the stem returns as the spring expands. Cherry claims that this innovation lowers the volume of their switches by around 30%.
For do-it-yourselfers, installing an o-ring on each switch can significantly lower your keyboard's volume. O-rings are small silicone rings placed around the stem of a key. With O-rings installed, the noise is greatly reduced when a keystroke bottoms out (is pressed all the way into the keyboard's body plate).