Designs, whatever we're working on, have unique rhythms. Much of the time, music, at least what I listen to the most, is a distraction. But other times, something syncs with the text, images, pages and spaces I work on. From time to time, I go on jags, listening to similar or the same pieces over and over. Recently I discovered Japanese rap. The Japanese are good at mimicking American (and probably other) cultural affectations. Often, their music is accented with English words and phrases mixed with their own language. What I like is the music, rhythm and pacing of rap without words I can possibly hope to understand. Additionally, the unusual pronunciation and juxtaposition of syllables wash over me like a subtheme to the bass beat.
Another designer, late twenties, in our office, likes to play Medeski, Martin and Wood on his ipod while building Flash segments on websites.
Whatever works for you, I would like to hear about it.