The correct answer is option 1: pressure. The amplitude of sound waves refers to the maximum displacement or intensity of the sound wave from its equilibrium position. In the case of a sound wave, this refers to the maximum pressure variation caused by the wave. Therefore, the amplitude of sound waves is measured in units of pressure.

Option 2, distance, is incorrect because while sound waves do propagate through distance, the amplitude specifically refers to the pressure variation rather than the distance traveled by the wave.

Option 3, time, is also incorrect as the amplitude is not directly related to time but rather the intensity or pressure of the sound wave.

Option 4, speed, is also incorrect as the amplitude is not related to the speed at which the sound wave travels but rather the pressure variation caused by the wave.

In summary, the amplitude of sound waves is measured in units of pressure.