
I was recently talking with a videographer friend. A great shooter with years of experience. Discussing a problem with low audio levels in editing, I asked if the audio was normalized and was compression used? The answer was no to both, and it turns out that this person was not really familiar with compressions, and how to use it.
In my experience, they're not alone. Good audio is crucial to video production. If the volume jumps dramatically between scenes, or some scenes are inaudible, the perception of your work is affected, and not in a good way either. Audio is so important that great sound can even improve the perceived quality of mediocre video.
First of all, normalize your video. This will justify the sound level so that the loudest peak is around 100%. Typically, right clicking on the audio event and selecting "normalize" will do the trick. Different editing apps vary on this, and may be adjustable. Secondly, set your track level so that the loudest portion doesn't exceed 0 db. In my example, I'm using Sony Vegas, but these steps are similar in most video and audio apps out there.
Apply a compressor plugin to the audio track, or as a non-realtime process to the audio event. In Vegas a compressor is already applied to the audio track, but it's set to a zero ratio (or amount) by default. Increase the amount (ratio) to around 3.5 to 4. Lower the threshold while playing the audio until the reduction indicator just begins to read about 2 or 3 db. The overall volume will usually decrease, unless you have auto gain compensate enabled. This is to be expected.
What's happening here is that when the audio level exceeds the threshold, it's reduced in volume by 4 times, or whatever the ratio is set to. This allows you to smooth out the very loud portions of the audio, and increase the quieter portions. And most importantly without exceeding zero db. This is to be avoided at all costs!
Experiment with threshold and ratio, and listen to the results. Don't get too carried away, or you'll have a dead one volume sounds as does virtually all pop music. I use third party compressors in most cases.
Waves is one such provider of audio plugins. There are also some freebies out there, both in DirectX and VST versions.