To determine what happens to the wavelength of a sound wave when its frequency increases while its speed remains constant, we must consider the fundamental relationship between the speed of a wave (\( v \)), its frequency (\( f \)), and its wavelength (\( \lambda \)). This relationship is given by the wave equation:

v = f \cdot \lambda

In this equation:

• v is the speed of the wave in the medium.
• f is the frequency of the wave.
• \(\lambda\) is the wavelength of the wave.

Since the speed \( v \) of the sound wave in the medium is constant, any change in frequency will inversely affect the wavelength. Let's analyze the situation step by step:

1. If the frequency \( f \) increases, then according to the wave equation, to keep the product \( f \cdot \lambda = v \) constant, the wavelength \( \lambda \) must decrease.
2. This occurs because the speed \( v \) is a constant value for the wave traveling in a given medium, and when the frequency goes up, the product of frequency and wavelength needs to remain the same constant value.

Thus, the correct option is: Its wavelength decreases.

To summarize, when the frequency of a sound wave increases and its speed remains unchanged, the wavelength decreases, as confirmed from the fundamental wave equation.