It appears as if the Dolby AC-3 patent may have expired:
http://www.cnx-software.com/2017/02/10/dolby-digital-ac3-us-patent-has-expired-on-february-1-2017/
http://www.cnx-software.com/2017/02/10/dolby-digital-ac3-us-patent-has-expired-on-february-1-2017/