It is so hard to know if you're infringing on a patent until some Patent Troll company serves you with papers to appear in Marshall, TX, Texas Eastern District Court. But the people at Xiph have done patent searches to try to be sure they are not infringing. That was their primary motivation in the development of Vorbis to get out of the licensing mess that was MP3/AAC. FLAC, while not developed initially by Xiph, has also been researched once it was brought into the fold.
Then again, since all codecs do is mathematical transforms of data which could also be performed in a person's head, they may not be patentable at all with the Bilski ruling.
