Sound is Important
When making a digital video film you want to use sound as another element to help communicate the story and mood of the project. Each drop of rain on a window or bark of a dog has a unique sound that tells more than simple images do. To accomplish this you may want to utilize sound effects, which are sounds that are put in during post-production, that you were not able to get when you were out recording.
Catalogue What You Need
To record you own sound effects you are going to have to decide not only what sounds you need, but also the context in which they will appear in the final video. For example, a phone ringing in a bedroom sounds different than a phone ringing in a giant concert hall. Make a list of the sources of the sounds, the size and nature of their spatial location, and the events that are surrounding them. Decide if they need to have an echo around them, or if they need to have a special length that may not be normal for the source.
For the sounds of small electronic devices, or other sounds that can be mimicked with small materials, it is best to record in a sound booth. This is where you can get the cleanest possible sounds without any outside noise. If you do not have access to one of these you can construct one easily at home. If you are recording on a home computer you can layer the walls with egg crates, mattresses, and blankets, to further insolate it. Try to use a high-quality microphone for these recording, such as an omnidirectional cardiod microphone for larger sounds and a ribbon microphone for smaller and more delicate sounds.
If you need to get a sound that you cannot control or recreate in a sound booth you need to get it from the actual environment. Take a high-quality digital voice recorder around with you and capture what you need. This may include chatter on a bus, the sound of a lawn mower starting across the street, or the noise of passing traffic. Whenever you capture one of these sound effects make sure to also record the background sounds of the area so you can edit those out in whatever sound mixing program you are using.
Once you have them all recorded, begin importing them into your editing project. Make sure to place them carefully, and that they always sound as if they were recorded at the actual location. There is nothing worse than having a sound effect sound artificially added. Try using audio transitions and effects to make it sound more natural, and lay over background noise and musical tracks beneath it to take the focus away from the sound.
Find Stock Sounds
This can be a time consuming and difficult process, so if you have the opportunity to get a package of sound effects at a reasonable price that should be your first priority. Remember, every choice you make should be well considered and the final video is meant to flow seamlessly. This is true for audio as much as it is for video.