Why does the pitch go up when you tap the bottom of your stirred cappuccino?

I've noticed and wondered that myself, when I'm stirring my coffee, cappuccino, or tea: as the sugar dissolves, the pitch of the sound of the spoon scraping on the bottom of the cup goes up. My son suggests it is probably due to the change in the propagation of the sound waves at the bottom of the cup, owing to the change in the density of the liquid they pass through as the sugar dissolves. At first, in that point of the cup the density is sludgy and full of particles, but then it gradually becomes lighter and more liquid.