You might have visited services where the volume between clips differs a lot. This can be very irritating as one clip is barely audible and the next screamingly high. Television had this problem with commercials but solved it with a loudness standard several years ago. Nowadays almost all web services have adopted loudness standards. This is great but learning all about loudness standards and how to process audio for certain platforms might not be everyone's biggest interest. In voicemachine we process all uploaded audio for consistent loudness. We also use a leveling algorithm that will adjust the audio so that the perceived level is equal even on less loud parts. This way you can listen to all demos and audio files in voicemachine at the same level regardless of their original level. We also deliver all approved recordings in 3 different dynamically mastered loudness standards plus the originally uploaded audio (unprocessed). This makes it very easy to produce consistent and loudness correct videos for the web with no technical knowledge about audio.
This is how to do it.
For web videos. Download the -16LUFS voice-over audio file and import it into your video editor. Do not make any changes in volume on this file. The -16LUFS voice-over track will be the anchor element around which FX and Music are mixed. Just add music and effects, making sure you can hear the voice-over loud and clear all the time. Never adjust the VO track in volume. You might have to add a little eq to the voice-over track to make it blend with the music/effects but this is easily done with the help of any video editor. If your video editor supports it, add a limiter at the master audio output and set the output to -1 dB in the limiter as a safety margin. The audio master shoud be at 0dB. This way your web videos will always have the same perceived level and will automatically be in the ballpark for the loudness standards of most web services such as YouTube, Vimeo, Social media uploads, etc (normally -16 to -14LUFS). You don't need any real technical skills to do this..:-)
To be able to produce commercials for broadcast there are other standards where the margins are much more critical so that will be handled in another article.
Good luck with your video production!
Special thanks to Alan Pryke and Frank Sanderson for proof reading.