FAAD2 is an open source MPEG-4 and MPEG-2 AAC decoder, it is licensed under the GPLv2 license.

www.audiocoding.com/faad2
github.com/knik0/faad2
faac.sourceforge.net
rarewares.org/aac-decoders
en.wikipedia.org/wiki/FAAC#FAAD2_decoder