In the "Meet the Soundtrack" sequence, that's not an equalizer bar, that's a fanciful depiction of the squiggly line of the optical soundtrack printed on the film.
http://en.wikipedia.org/wiki/Film_sound
Look at the picture in the above link. The two squiggly lines on the right of the picture are the optical soundtrack (in stereo, not the single squiggly line of the mono soundtrack they depicted in the film). The width of the peak correspond to the loudness of the sound; how often the peaks occur, the frequency.
By the way, there are three digital soundtracks on the film frame in the picture, too.
Another popular film that used the Flower Duet was True Romance. It's used in the scene where Clarence's father (Dennis Hopper) refuses to tell the Sicilian consigliere (Christopher Walken) Clarence's whereabout, and the consigliere kills him.
http://en.wikipedia.org/wiki/Film_sound
Look at the picture in the above link. The two squiggly lines on the right of the picture are the optical soundtrack (in stereo, not the single squiggly line of the mono soundtrack they depicted in the film). The width of the peak correspond to the loudness of the sound; how often the peaks occur, the frequency.
By the way, there are three digital soundtracks on the film frame in the picture, too.