Final answer:
The amplitude of the sound wave increases when the source's speed matches the speed of sound because of constructive interference, leading to a sonic boom.
Step-by-step explanation:
When the speed of the source matches the speed of sound, the phenomenon that occurs is known as the creation of a sonic boom. This happens due to constructive interference of the sound waves. As the source moves at the speed of sound, it continues to emit sound waves that pile up and overlap at the same point because the source is moving as fast as the sound itself. This results in a sudden increase in pressure and amplitude in the form of a shock wave, which we hear as a sonic boom. So, when the source matches the speed of sound, the amplitude of the sound wave does not decrease, it actually increases due to constructive interference.