When powdered hot chocolate is stirred into a cup of milk, the pitch produced by tapping on the mug changes drastically as the powder is stirred in. This phenomenon is often called the hot chocolate effect, but it can also be observed with other drinks such as instant coffee or cold beer.
In every case, the change in pitch is caused by the presence of air bubbles in the liquid. These bubbles are stirred into the mixture along with the powder ( giving the hot chocolate a thicker, foamy texture). However, since the speed of sound is much higher in water (or milk) than in air, these air bubbles lower the effective speed of sound in the hot chocolate. This in turn lowers the pitch at which the mug resonates when tapped.
Over time the gas bubbles dissipate, accounting for the steady rise in the pitch of the tapping after the stirring stops. Stirring the hot chocolate again reintroduces the bubbles, starting the effect all over again. You can even hear the pitch lowering as you stir by listening to the sounds made as the spoon accidentally hits the side of the mug.