Wikipedia reckons that MP3 is now patent free:
Quote:
The basic MP3 decoding and encoding technology is patent-free in the European Union, all patents having expired there by 2012 at the latest. In the United States, the technology became substantially patent-free on 16 April 2017 (see below). The majority of MP3 patents expired in the US between 2007 and 2015. In the past, many organizations have claimed ownership of patents related to MP3 decoding or encoding. These claims led to a number of legal threats and actions from a variety of sources. As a result, uncertainty about which patents must be licensed in order to create MP3 products without committing patent infringement in countries that allow software patents was a common feature of the early stages of adoption of the technology.
The initial near-complete MPEG-1 standard (parts 1, 2 and 3) was publicly available on 6 December 1991 as ISO CD 11172.[68][69] In most countries, patents cannot be filed after prior art has been made public, and patents expire 20 years after the initial filing date, which can be up to 12 months later for filings in other countries. As a result, patents required to implement MP3 expired in most countries by December 2012, 21 years after the publication of ISO CD 11172.
An exception is the United States, where patents in force but filed prior to 8 June 1995 expire after the later of 17 years from the issue date or 20 years from the priority date, a lengthy patent prosecution process may result in a patent issuing much later than normally expected (see submarine patents). The various MP3-related patents expire on dates ranging from 2007 to 2017 in the United States[70] Patents filed for anything disclosed in ISO CD 11172 a year or more after its publication are questionable. If only the known MP3 patents filed by December 1992 are considered, then MP3 decoding has been patent-free in the US since 22 September 2015 when U.S. Patent 5,812,672 expired which had a PCT filing in October 1992.[71][72][73] If the longest-running patent mentioned in the aforementioned references is taken as a measure, then the MP3 technology became patent-free in the United States on 16 April 2017 when U.S. Patent 6,009,399, held by[74] and administered by Technicolor,[75] expired.
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https://en.wikipedia.org/wiki/MP3#Li...nd_legislation
If the last remaining US patent did indeed expire on 16th Apr 2017, this may explain the timing of this: Fraunhofer are no longer issuing licences because they no longer hold any rights to the technology.