TagLib is a library for reading and editing the metadata of several popular audio formats. Currently, it supports various metadata containers such as ID3v1, ID3v2 and Vorbis comments for MP3, MP4, AAC, Ogg, Opus, FLAC, Speex, APE, MPC, WavPack, WAV, AIFF, TrueAudio, Matroska, WebM, ASF, WMA, DSF, DFF and tracker (MOD, XM, S3M, IT) files.
TagLib is distributed under the GNU Lesser General Public License (LGPL) and Mozilla Public License (MPL). Essentially that means that it may be used in proprietary applications, but if changes are made to TagLib they must be contributed back to the project. Please review the licenses if you are considering using TagLib in your project.