CoreAAC is completly free and open-source. It is based on FAAD2 and released under the GNU GPL licence. For licence and patent reasons there are no compiled versions on this website. But you´ll find them at Rarewares.

AAC means 'Advanced Audio Coding', and in the beginning it was also called MPEG-2 NBC for 'Non-Backwards Compatible' as opposed to the MPEG-1 and MPEG-2 BC (with 5.1 channels) standards. It is now considered to be the actual 'state of the art' in general audio coding and the natural successor of MPEG-1/2 Layer III / MP3 in the new multimedia standard MPEG-4 that uses MP4 as the container format for all kinds of content.
AAC is able to include 48 full-bandwidth (up to 96 kHz) audio channels in one stream plus 15 low frequency enhancement (LFE, limited to 120 Hz) channels and up to 15 data streams. Besides it has further multi-language capacities.