This one is even harder than the punk rock one in pinning down a specific POD, but just as a stab, let's say Nile Rodgers sues the pants off the Sugar Hill Gang over "Rapper's Delight" in 1979. The rule that emerges from this and other court cases is that DJ's can "toast" or "rap" over prerecorded tracks at events (dances, parties, etc.) but that if it makes it to a recording, they're violating the copyright.
The bigger question is how dance music/R&B evolves without rap/hip-hop?