Making sure your audio can be heard easily and clearly is important. One of the best audio editing tools available to us is called compression. With a compressor, we can even out the volume of certain tracks in our recordings, or even the entire mix. One primary reason for this is to prevent the listener from having to turn up the volume to hear the soft parts, but then also having to turn the volume down to keep from getting their ears blasted out during the loudest bits.
This loudness leveling has another...ahem...benefit. Since it turns down the loudest bits without turning down the perceived average loudness of the entire song (or any audio content, like voice-overs, podcasts, etc.), some blank space is left between the former loudest bit and 0dB, the loudest possible level, before distorting. "Well..." say the producers, "we can't have any blank space on the final track. That would mean the song isn't as loud as it could be." So now the entire song's average loudness can be increased by raising ALL the levels until the loudest one is just barely below 0 dB.
Here is an interesting article about "the loudness wars," as they are sometimes called, and how the search for ever-louder mixes can crush the life out of the music.
The original article is here: http://www.prosoundweb.com/article/the_loudness_wars_a_graphic_look_at_hypercompression/