Dr Dre and Jimmy Iovine are being sued by a former business partner who co-designed their Beats headphone range.
Noel Lee, whose firm Monster helped launch Beats by Dre in 2008, says his company was betrayed and its technology "pirated".
He alleges he "lost millions" after Dre and Iovine "improperly took control" of Beats through a "sham" transaction.
After severing ties with Monster, Beats was purchased by Apple for $3 billion (£1.9 billion).
In legal papers filed in California, Lee alleges rap producer Dr Dre was barely involved in creating the headphones that carried his name.
But the range quickly became a highly desirable brand with celebrities and music fans, and Dre's endorsement was key to its success.
When the firm was sold to mobile phone manufacturer HTC in 2012, Dre and Iovine made $100m (£66m) each, Forbes estimated.
But Lee claims the deal forced him to cut his share in the company from 5% to 1.25%. He says he then had no other option but to sell his remaining stake for $5.5m (£3.6m) near the end of 2013.
His legal case describes the HTC deal as "fraudulent" and "a sham" intended to shift ownership of Beats to Dre and Iovine.
HTC later sold its shares back to the Beats.
Lee says if he'd still had his 1.25% stake, he would have received more than $30m (£19.8m) in the Apple deal.
His original 5% stake would have been worth around $150m (£99m).
A statement from Monster's law firm Cotchett, Pitre & McCarthy says that Dr Dre and Jimmy Iovine "made millions off the work of Monster and Lee, while the plaintiffs lost millions".
There's been no response yet from Dr Dre's lawyer - or from Beats.
It's the second lawsuit against the firm since Apple bought it last year.
The first was brought by the Bose Corporation over patented technology for cancelling noise in earphones.
That case was settled under undisclosed terms in October.
Dr Dre and Jimmy Iovine are also being sued by David Hyman, who sold his music streaming service MOG to Beats in 2012.
That court case, filed shortly before the Apple deal was sealed, is taking place now in Los Angeles.
Jimmy Iovine is the co-founder of Interscope Records, a rap music pioneer that branched out to include acts like Lady Gaga and U2.