Human hearing capacity is in range of 20Hz to 20 KHz and the ultrasonic sound being monitored is sound waves that occur in the 20KHz to 100 KHz range, making it impossible for early detection of a developing problem without the competent use of an Ultra Probe. Ultra-probe is sensible enough to detect pinhole leakage as low as 1psi.
Ultrasound is a very good go-no-go tool for predictive maintenance. It can give you a very early indication of developing faults. Once you detect issues with ultrasound you can further investigate the vibration spectrum for detailed analysis.
It is also a very good diagnostic technique to identify lubrication issues. Lubrication issues solved in time can prevent further damage to bearings and rotating parts.