Here's the story:
Company buys the rights to Winamp, tries to get the community to do their dev work for free, fails. That's it.
The 'Winamp source license' was absurdly restrictive. There was nothing open about it. You were not allowed to fork the repo, or distribute the source code or any binaries generated from it. Any patches you wrote became the property of Llama Group without attribution, and you were prohibited from distributing them in either source or binary form.
There were also a couple of surprises in the source code, like improperly included GPL code and some proprietary Dolby source code that never should have been released. The source code to Shoutcast server was also in there, which Llama group doesn't actually own the rights to.
This was a lame attempt to get the community to modernize Winamp for free, and it failed.
Of course many copies of the source code have been made, they just can't be legally used or distributed.