libsndfile
The C library that reads and writes audio files for half the open-source audio world, so no program has to parse WAV headers by hand.
Australian developer Erik de Castro Lopo started libsndfile in 1999 and released it under the LGPL. The API is deliberately boring: open a file, read or write frames as integers or floats, close it. Behind that sit WAV, AIFF, FLAC, Ogg Vorbis and Opus, plus dozens of obscure formats from old samplers and workstations, with every variant of byte order and header they ever shipped. MP3 support only arrived in 2022, once the patents had safely expired.
It sits underneath Audacity, Ardour, Csound, SuperCollider and PulseAudio, usually without the user knowing. The same author wrote libsamplerate for sample rate conversion. Like a lot of critical open-source plumbing, it has at times been maintained by more or less one person.