I was sitting at my
lab bench working with my fruit flies one day. The guy who sits across the bench had a
radio turned on. I was bored, so I was trying to listen to the music while transferring the flies from one tube to another. The problem was that the radio was playing rather softly and it was turned away from me. I strained to listen to the song, but I couldn't hear it. All of a sudden, I picked out a single line from the song. I recognized that line and immediately after that, it seemed like the radio had gotten louder because I could suddenly hear the singer over the music.
In other situations, I've noticed that if you are trying to listen to people talking softly on the other side of a door, it sounds like indistinct mumbling until you actually pick out a few words and then it magically becomes easier to understand what they're saying. It's especially true if they're reciting something familiar to you. That leads me to conclude that the perceived increase in loudness has something to do with the listener's "anticipation" of the next sound he will hear (the next line of the song or passage). But there have also been times when I've been listening to jazz and it all seemed to blend well, but then I picked out the string bass plucking away and then that was all I heard for the rest of the song, even though I wanted it to blend like it had before and the song was unfamiliar to me. I'd be curious to know if there have been studies done about this effect and if everybody else has experienced this.