SRS iWOW for iTunes
This plug-in for any Apple or Windows laptop helped overall volume and many sounds, but it also occasionally produced interference on our MacBook Pro's speakers. It turned out clearer bass tones, and emphasis on extreme high notes in Music and Movie modes made dialogue and instruments like cymbals stand out clearly. Unfortunately, some music presets (for, say, rock, jazz or classical) added a hissing sound, and we heard crackling when we pushed volume and enhancement settings to the max.
$70 (for software); srslabs.com