Methods, apparatus, and storage media to build a music metadata library from music collections stored on a plurality of client computing devices. A music server may receive, from a first client computing device of the plurality of client computing device, a unique identifier for a track stored in the first client computing device. The music server may determine if the track is a known track corresponding to acoustic metadata that is already contained in the music metadata library. When the track is not a known track, the music server may send a message indicating the track is a new track to the first client computing device. The music server may then receive acoustic metadata for the track from the first client computing device and store the received acoustic metadata in the music metadata library.