First two questions are certainly difficult because even the experts do not have convincing explanations. Explanation of the third question, however, is quite simple. The process of hearing commences with the vibration of the eardrums. In order to allow the eardrums to vibrate freely it is essential that there is equal air pressure on both the sides of the eardrums. So Nature has arranged for as much air pressure on the inner side of the eardrums as the air pressure prevailing on the outer side to keep the eardrums neutral. When a person yawns, the internal muscles of the ears contract increasing the pressure from the inner side which in turn pushes the eardrums somewhat outward. Now in the outward dilated condition of the eardrums the sound waves are not able to vibrate the eardrums and so the sound is not heard till the eardrums come back in their original position.
The reason for yawning and infecting others into yawning is still a subject of research, but some aspects have been understood in the course of research on the subject. When a person is tired at the end of the day his muscles relax and the rate of breathing also slows down automatically. An adult person requires about 6 liters air per minute. About 15% of this quantity is consumed by the brain alone. It takes about 0.85 liter blood per minute to feed the oxygen in this air to the nerve cells of the brain. As the rate of breathing slows down the brain feels suffocation for the want of oxygen and sends ‘public’ signal to the lungs to take a deep breath in the form of yawn. The yawn which follows lasts about 5.5 seconds. And if the brain is not satisfied with one yawn then some more yawns may follow. Repeated yawn are also a signal of approaching sleep. When asleep the body does not require much oxygen and also gets the rest it needs. Therefore, one does not yawn as much during the day time as late in the evening or in the night except when studying or compelled against will to do some boring task!
The researchers have not arrived at the conclusive explanation of why yawning is infectious. However, there is an interesting hypothesis. The control center for yawning in the brain sends signals for yawn on identifying fatigue, inertia, sleepiness or boredom. When we see somebody yawning this visual hint stimulates our brain to identify our own tiredness etc. if it has not already done and to send signal to lungs for yawning – and there it is! Yawn…!
Finally, an interesting piece of information: Whether during the day time or night most of the paratroopers of the army or the adventurous sky divers invariably yawn once or twice before jumping out of the plane to exercise muscles of the ears to be in readiness for low air pressure outside the aircraft.