Usually the medium is air, but liquids generally work even better. The pressure waves are generated from some type of mechanical disturbance. Sound is a transfer of power as well. Mechanical energy is being converted to a wave form that radiates energy away from the disturbance. Mechanical vibrations become vibrating pressure waves, transferring energy to the medium and to objects that the wave contacts.
Human hearing is limited.
If the vibrational frequency is too fast, too high a frequency, we can't hear it. This is ultrasound, vibrations too fast for us to hear. Vibrations Too Fast To Hear. Human hearing can't go beyond about 18,000 vibrations per second, or 18 kHz. There are mammals that can hear well above this.