Any vibrating matter is a source of sound. If there's a medium around it, then the vibrations can set up waves in the medium and carry their energy away from the source. If their frequency and amplitude happen to be within the detection capability of some detecting device, such as an ear, AND some of the waves happen to strike the device, then their presence will be detected as sound. (they'll be "heard")
The sound waves are generated by a sound source, such as the vibrating diaphragm of a stereo speaker. The sound source creates vibrations in the surrounding medium. As the source continues to vibrate the medium, the vibrations propagate away from the source at the speed of sound, thus forming the sound wave.
Only the horizontal component of the tension force is performing work on the crate. This component has magnitude (50 N) cos(30°) = 25√3 N, so the work done is
