The headphones became tremendously popular among young people, fashionistas and celebrities.
The defendants "made millions off the work of Monster and Lee, while the plaintiffs lost millions," according to a statement from Monster's law firm Cotchett, Pitre & McCarthy.
It's the second lawsuit against Beats since Apple bought it last year for $3.2 billion.
The first was brought by the Bose Corporation over patented technology for canceling noise in earphones. That case was settled under undisclosed terms in October.