> In the United States, the technology will be substantially patent-free on 31 December 2017 (see below). The majority of MP3 patents expired in the US between 2007 and 2015
(2)Term.—
Subject to the payment of fees under this title,
such grant shall be for a term beginning on the date
on which the patent issues and ending 20 years from
the date on which the application for the patent was
filed in the United States or, if the application
contains a specific reference to an earlier filed
application or applications under section 120, 121,
365(c), or 386(c), from the date on which the
earliest such application was filed.
(3)Priority.—
Priority under section 119, 365(a), 365(b), 386(a),
or 386(b) shall not be taken into account in
determining the term of a patent.
For this patent, there is a chain of specific references to earlier filed applications, but I'm not sure which of those, if any, are under section 120, 121, 365(c) or 386(c) and so affect the expiration date of the patent.
This patent, for example, doesn't expire until near the end of 2017: https://www.google.com/patents/US5703999?dq=5,703,999
Edit: This is supported by the wikipedia article: https://en.wikipedia.org/wiki/MP3#Licensing.2C_ownership_and...
> In the United States, the technology will be substantially patent-free on 31 December 2017 (see below). The majority of MP3 patents expired in the US between 2007 and 2015