There is no reason to believe that having an ultra sound would harm the hearing of an unborn child. Their hearing responds to no higher frequencies than a child's hearing. Having several ultrasounds is no different then having a dog whistle blown in your ear.
The best way to understand why it won't affect your child's hearing is to understand the way the ear works. As sounds travel from your outer ear down your ear canal the pressure waves impact your ear drum. As your ear drum vibrates it transfers the signal to several bones in your inner ear, the Incus, Malleus and Stapes. These bones then transfer the vibrations to the Cochlea which which then transfers the sound wave to a nerves that carry the signal to your brain.
The ear drum itself is simply a taut membrane of skin. It has the ability to vibrate at frequencies of about 20 Hz to 20 kHz at the maximum. The membrane can be throught of as a damped oscillator, and as such does not vibrate or respond to frequencies that are out of range. Ultrasound by definition is considerably beyond the hearing range of humans, and as such should not affect the delicate parts of the ear of an unborn child.
Imagine if you were driving a car down the road and you felt the bump of a pot hole. This meant the hole was wide enough, or has a low enough frequency, that you could feel it with your car. Now when you are driving down the same road it is not perfectly smooth, there are small crevices and uneven patches between the stones that make up the asphalt. You cannot feel those as you are driving because those bumps and ridges are at too high a frequency and are beyond the frequency range of the tires to transfer to the passengers. This is the same way that the frequency of the ultrasound is too high for the parts of your ear to transfer to your brain, or even vibrate themselves.
There is really no need to worry, you aren't really blasting your child's ears with rock music each time you get an ultrasound. Now if you were to go to a rock concert and stand in front of the speakers while you were pregnant, then you could probably not only damage your child's hearing, but also other developmental issues. Low frequency is probably much more harmful because it can cause entire tissues to vibrate. If you have ever been to see fireworks and feel the concussion against your chest when the firework explodes you are feeling extremely low frequency pressure waves. The same is true when you feel the bass from a loud rock concert thumping in your chest.