The band BIS, who are known for creating the "Powerpuff Girls End Theme" also created the song "Statement of Intent" which can be heard in the game.
There is an unused song titled "ending.adx" which, as the name implies, could have been used as the credit music. In Jet Set Radio, the ending medley ended on this song, also supporting the theory.
Collecting all of the Graffiti Souls reveals a hidden message in the graffiti creator. The selection box for each size category shows a portion of the message, and together they read "Be a master of the streets."