This is the violin that Frey uses. He is usually seen with his violin in most of the concept art, although he hasn't been seen with it in the comic.
Frey regularly plays his violin when he needs to think about the next strategy to use in cyber warfare against a totalitarian regime.
Frey's violin is a black electric violin. When Frey is playing his violin he is shown with it plugged into his headphones, thus making it virtually silent to everyone else around him.