Stephen Marley comes from music royalty. Sean Garrett has written and produced smash hits for Beyoncé, Usher and Ciara. Even so, when they and hundreds of lesser-known names in the world of hip-hop and rap needed cash, one Wall Street figure emerged as an unlikely source behind the financing: hedge-fund titan Jamie Dinan. In a few short years, Sound Royalties, a West Palm Beach firm started within a unit of Dinan’s York Capital Management, has become ubiquitous in the music industry.