Sound is a wave, and waves need some sort of matter to move through (the scientific term for waves moving is "propagating"). Sound moves through more than just air. If you've ever put your ear on the wall or something solid, you know that you can still hear things that come through. Also, since waves need a medium (matter to move through), you can probably figure out that in space, there are no noises! Some films don't take this into account, so next time you watch a film based in space, see if you can spot whether or not the director made sure to remember his science classes!

So, now that we have some idea of what sound needs to propagate, we can ask further: why is this important? Why would humans and other animals evolve to detect sound waves moving in the air? There is no simple answer that is complete, so be aware that there are lots of factors that have influenced the way evolution has progressed. If we think critically, though, we can come up with a few good reasons why animals would find sound detection useful.

First, think about hunting. In the wild, being able to hear predators is essential for survival. Eye sight also helps, but sound can travel through small spaces and around corners very easily, so detecting moving, breathing, or other sounds is very useful. It is useful for both parties: the hunter and the hunted. Imagine if you are alone in a forest, and it's night time. Without your sense of sound, it would be extremely difficult to navigate or stay out of harms way.

Secondly, finding running water is much easier when you can hear! Water is an essential element of life. Human bodies are almost two-thirds water! Without it, no animal would survive.

Lastly, communication. The first two examples were very survival-based. But once our basic needs are met, there are other advanced ways that animals interact. Humans have developed spoken language because we have the tools to send a receive messages with waves! That's an incredible feat, and a very useful skill. As we said before, sound can travel very far, and it moves outward from the point that created it. This lets animals warn others of predators, or make them aware of food or water in the area. This reason is by far the most broad, since communication exists in so many forms. From survival communication to social communication, both have been integral in human and animal development.

Thanks for the great question, I hope this answer gets you thinking about the many ways sound has influenced our planet!

Sound is biologically important because vibrations carry energy and thus information over distance reliably, which is otherwise difficult for living systems to do (except for vision, of course, but sensing light is if anything even harder than sound, ironically enough). For this reason, hearing and vibration sense are extremely important senses for living organisms. Vision is even more important for many animals, because light is even more effective at transferring information, despite being even harder to detect.

This is an interesting question. I think there are many possible answers, because of how open-ended you've left the question. For instance, sound is important to humans and many other animals for communication, as well as for awareness of surroundings for safety.

Sound played an important role in the historical development of languages as a means of communication. There are some words that are as old as 15,000 years old that are relatively unchanged across many languages in the world! Here is a link if you are interested:

Even for babies, sounds such as different kinds of crying are necessary for communicating needs to their caretakers. Furthermore, it is in those first few years of a human's life that an individual organizes different types of sounds in their minds, not only in the way of words, but as a way of expressing emotion through intonation. I think a very explicit way we express emotion is through music, but even apart from music, people use inflection in their voices to convey feelings of joy, anger, fear, contentment, etc.

Sound is also important for safety. Think about reflexes people or animals have to very loud, crashing noises. Sound is a tool that organisms use to stay aware of their environment as a way to improve their chances of survival.