A lot of what you see in video is what you hear. We think of video as a visual medium, but it is very dependent on its auditory component. Since sounds and images were married together in video people have come to have high expectations for audio when they watch video, and you should be prepared to deliver. It often gets forgotten because of the way we talk about video: "did you see that?" "Hey watch this," "How many views does it have?" But audio is one of the most important things to consider when making video.
People are far more forgiving of bad video than bad audio when they watch. Think about some of the videos you have seen on the web. Shaky videos can get millions of views, but videos with bad audio get destroyed in the comments. This is why when you are putting your video together, either shooting or editing, you must pay particular attention to the your audio acquisition, audio mix, sound effects, and all things heard.
First, let's take a look at exactly how important audio is to the viewing experience. Let's look at Planet Earth, probably the most successfully nature documentary of all time. People love this program. They love it because it gives them an intimate experience with animals and plants from around the world. What you may not realize watching Planet Earth is how much of that intimacy is driven by audio -- most of which is fake.
You see the extent that the producers go to ensure that the audio is cinematic and helps drive the story, and this is something that you should be doing as well. You may not have the time or budget to create sounds, but you should make sure that there is an auditory experience that matches and compliments your visuals.
This goes for all kinds of videos, whether you are making a nature video, travel video, fiction short, food video, or whatever, you have to make sure you get the sounds right and crisp and use them throughout your story. We say that when you are making a video you are the viewer's surrogate into that world, so the sounds of that world are very important. If you are making a travel video that could be the sounds of the streets where you are or mountains you are hiking, for a food video this may be the sound of frying food and mixing bowls. Whatever the video is you have to make sure to record the sounds (or use sound effects afterwards) that create the world.
Mostly, this is done when you are recording. Make sure that you mic the space to hear these sounds whether it be a shotgun on your camera or a lav mic on your subject. Also take time when you are recording to just record these sounds to use later. If you are on a hike of a mountain, take a moment to record the sounds of the animals, wind and anything else that you can use later.
It's easy when making video to get hung up exclusively on the visuals, but remember that when the viewer sits down to watch, they will be listening too!