Mp3tag is an Objective-Caml library for reading and writing ID3v1 and v2 tags in MP3 files. It includes mp3tag tools, which can be used to modify tags in MP3 files or rename files according to their tags, and a cddbtag tool that modifies tags by using a CDDB server to retrieve information about the track.
The FAAC project includes the AAC encoder FAAC and decoder FAAD2. It supports several MPEG-4 object types (LC, Main, LTP, HE AAC, PS) and file formats (ADTS AAC, raw AAC, MP4), multichannel and gapless en/decoding as well as MP4 metadata tags. The codecs are compatible with standard-compliant audio applications using one or more of these profiles.