Many artists, both synesthetes and non-synesthetes, work with visualizing sound and music. These are some projects that I have found fascinating and inspiring for my personal work on visualizing death metal.
Julien Lassort and Matthieu Burlot, a Parisian creative team better known as Jul&Mat, created this spectacular (unofficial) video for Metronomy's "On the motorway."
The idea is simple and so effective, and the different graphics really bring out the melodies and rhythmns layered throughout the song. I really love the way they were able to show timbral textures through different colors, each in a different style. Absolutely stunning!