3GPP SA4: This working group of the standardization organization deals with codecs like enhanced aacPlus or AMR-WB+ used in mobile telecommunication.
AES Publications: All available articles and documents from the Journal of Audio Engineering Society with search tool including a CD about coding artifacts.
Apple: Overview of their implementation and contribution to the MPEG-4 standard.
Applications of Digital Signal Processing to Audio and Acoustics: Book by Mark Kahrs and Karlheinz Brandenburg.
Audio Research Labs: ARL is a commercial laboratory involved in subjective quality assessment methods like preparing and analyzing codec comparisons for 3GPP.
Audiocoding.com: The open source project FAAC includes an AAC encoder and decoder (FAAD2) with source code packages, developer mailing list, knowledge base, news section and user forum.
Coding Technologies: Company that invented Spectral Band Replication (SBR) and combined it with MP3 (mp3PRO) and AAC (aacPlus or HE AAC).
Digital Radio Mondiale: DRM is a world-wide initiative to use analog AM radio for digital sound and services implementing aacPlus at very low bitrates, often mixed up with Digital Rights Management for copy protection.
Doom9: Besides this forum for audio codecs there are others for container formats and video encoding in general with FAQs.
EE Times: "Meeting MPEG-4 advanced audio coding requirements", knowledgeable article summarizing the most important properties of AAC for hardware playback.
Everwicked.com: Comprehensive guides about DivX, XviD and MPEG4IP, a forum and other resources about digital audio/video coding and streaming.
Faac.sourceforge.net: The old Audiocoding.com forum is still available providing valuable content like FAAC compiling issues with its search tool.
Facial Animation: AT&T Labs develops a computer human interface using 2D and 3D talking head-and-shoulder models driven by a Text-To-Speech synthesizer (TTS).
Fraunhofer IIS: Information about the different parts of MPEG-4 Natural or General Audio (GA) from one of the inventors of these formats, see also their page about MPEG-2 AAC.
Hydrogen Audio: Forum about audio coding in general with two separate boards for AAC.
IBM Composite Media Group: Overview of their implementation and contribution to the MPEG-4 standard, especially for the Systems part.
Introduction to Digital Audio Coding and Standards: Book by Marina Bosi and Richard E. Goldberg.
ISO/IEC: Publicly available standards from the International Organization for Standardization including MPEG-4 with 14496-5 containing the Audio, Visual and Systems parts.
Machine Listening: Reading list for an MIT seminar, most of the mentioned publications are directly available on their web server or linked to Amazon's book section.
Mayah: Hard- and software company for professional broadcasting products with AAC/MP4 support.
Mobiledia: Reviews, articles and forum discussions about cell phones capable of AAC/MP4/3GP playback (also called "iTunes" format sometimes, use the site search).
MP4-SA Developer Tools: John Lazarro and John Wawrzynek from the CS Division, UC Berkeley provide tutorials, downloads, manuals and links related to the Structured Audio format.
Mp4-tech: The web archive of the public MPEG Industry Forum mailing list with many knowledgeable participants from companies like FhG, Apple and Microsoft.
MPEG Audio: Official site of this MPEG subgroup with FAQs for the different standards, publicly available reference documents and software.
MPEG LA: Patent pool administrator for the MPEG-4 Systems part of the standard including the MP4 file format.
MPEG-4 Structured Audio: Technical information and developer resources from the Machine Listening Group at the MIT Media Laboratory, the official homepage for this format maintained by Eric Scheirer.
MPEG4.net: Resource for news, information and products about MPEG-4, H.264, Windows Media, High-Definition, Streaming Media and related technologies.
Nokia developer forum: Platform to support the implementation of their software SDKs including AAC/MP4.
Principles of digital audio: Book by Ken C. Pohlmann.
QuickTime development: Apple offers technical FAQs, knowledge base and publicly available documentation files including the MOV file format.
Roberto's listening tests: Provides public group comparisons of several AAC codecs and other formats at different bitrates.
Ross Bencina Structured Audio: The writer of AudioMulch, free soundsynthesis software, offers a SAOL/SASL reference guide and decoder resources.
Speech Synthesis Examples: Many TTS demos in several languages from different systems with links to their homepages.
Structured Audio Open Group: The main goal of the SAOG was to implement Structured Audio (SA) in open source applications offering a portal for their users with forum and downloads of samples and software.
Telos Systems: Hardware manufacturer of the Zephyr ISDN codecs for radio and television broadcasters providing articles and brochures about different AAC implementations.
Text-To-Speech Synthesis: Bell Labs' / Lucent Technologies multi-lingual TTS demos and introduction to this coding method.
Tuner2: List of internet radio stations using aacPlus for their low bitrate streams, among them SomaFM.com and Boomer Radio.
Via Licensing: Patent pool administrator for AAC with FAQs and overviews of royalty fees for commercial implementations.