If you blow harder into a pipe, the property of sound that is most likely to change is the amplitude.
Amplitude refers to the magnitude or intensity of a sound wave. When you blow harder into a pipe, you increase the air pressure and energy being delivered to the pipe. This increase in energy causes the air molecules in the pipe to vibrate with greater displacement, resulting in a larger amplitude of the sound wave produced.
The frequency of the sound wave, which corresponds to the pitch of the sound, is primarily determined by the properties of the pipe itself, such as its length and shape. Blowing harder into the pipe may not directly affect the frequency unless it leads to changes in the pipe's resonant characteristics.
The speed of sound in a given medium, such as air, remains relatively constant under normal conditions and is not significantly influenced by blowing harder into a pipe.