Not really because their rights have not been violated, nothing was stolen from them. They were presented with a software product that had a limited license, and they accepted that. As far as they are concerned, the developer has fulfilled their contractual obligation to them; they were never offered a GPL license so they got exactly what they were offered.

The author of the GPL'd code however is another story. They wrote software distributed as GPL, Winamp took that code and included it without following the GPL. Thus that author can sue Winamp for a license violation.

Now if that author is the only one who wrote the software, the answer is simple- Llama Group pays them some amount of money for a commercial license of the software and a contract that this settles any past claims.

However if it's a public open source project, it may have dozens or hundreds of contributors, each of which is an original author, each of which licensed their contribution to the project under GPL terms. That means the project maintainer has no authority to negotiate or take payments on their behalf; each of them would have to agree to that commercial license (or their contributions would have to be removed from the commercial version of the software that remains in Winamp going forward). They would also each have standing to sue Llama Group for the past unlicensed use of the software.