Microsoft’s very-proprietary Windows Media Audio format has been reverse engineered as part of the FFmpeg project. Codecs from the FFmpeg project are licensed under the LGPL and are used by several other projects.
Now, the only hitch to building Windows Media Audio-compatible players is Microsoft ASF-related patent, which I personally believe is invalid based on prior art. But it seems hopeless for individuals or small companies to challenge it, and not a good business move for larger companies to challenge it. Proprietary formats suck, don’t they?