Answer
The sound comes from air molecules in the glass oscillating as the liquid being poured hits the surface of the liquid in the glass.
The equation for the fundamental frequency is $f = \frac{v}{4L}$, where $L$ is the length of the tube. As the glass fills up, the length $L$ decreases, thus increasing the frequency.
Work Step by Step
A glass with liquid is like a tube that is closed at one end. The sound comes from air molecules in the glass oscillating as the liquid being poured hits the surface of the liquid in the glass.
For the tube with one end closed, the equation for the fundamental frequency is $f = \frac{v}{4L}$, where $L$ is the length of the tube. As the glass fills up, the length $L$ decreases, thus increasing the frequency.
