One aspect of sound that allows for pianos and other instruments to play in things like concerts is that frequency is nearly independent of speed. If this were not true, people in the back of the audience would hear different pitches of sound as the people in the front of the audience and the performers would hear.
Pianos are specifically built as a kind of a large echo chamber which allows the sound produced from the strings to resonate and grow to produce a louder and more booming sound. Another unique thing about piano strings that helps with the sound is the fact that the strings are tuned and lengthened to a fundamental frequency which increases the sound created by the stings.