The "source" of the Matroska specification is an XML file hosted on the IETF working group's GitHub repository (although that table has not been updated since 2020). This table is also used to generate the semantic data used in libmatroska and libmatroska2.
Specification information is published concurrently in two locations: The Matroska website and the Internet Engineering Task Force (IETF) Matroska Media Container Format Specifications datatracker. Since 2018, the in-process specification for version 4 of the Matroska Media Container Format has been published as a sequence of Internet Drafts, available at Matroska Media Container Format Specifications datatracker. An early version of the specification dated January 11, 2009 authored by Alexander Noé is available on the Matroska site but it is not clear which versions are covered in this document.
For MP3: (1) MPEG-1: ISO/IEC 11172-3. Information technology -- Coding of moving pictures and associated audio for digital storage media at up to about 1,5 Mbit/s -- Part 3: Audio. (2) MPEG-2: ISO/IEC 13818-3. Information technology -- Generic coding of moving pictures and associated audio information -- Part 3: Audio.
These MPEG specifications describe the syntax and semantics for three classes of compression methods known as Layers I, II, and III. MP3 is Layer III. See list of ISO documents in Format specifications below; see also MPEG-1 and MPEG-2.