The invention, as shown by the system in FIG. 2, relates to a client-side content tracking system of media files, e.g. digital music files. Audio tracking—or indeed multimedia tracking—is shifted to a client-side perspective, with the client tasked with establishing use of a selected source audio track by tracking—and then reporting uplink to the server—at least one of: entry and exit points associated with playing of at least one of said musical sections in the identified source audio track, and how the identified source audio track was used, performed or manipulated at the client device. Server functionality is designed, having regard to the reported tracking data and its link to a unique identifier to permit the media file (e.g. source audio track) to be selected and/or identified, to store or relay—possibly in the context of a subscription service and billing regime for content use—tracking data related to use of at least a portion of the source audio track at or by the client device. In the context of audio, reporting of use at a client device can, in turn, cause streaming of related multi-media content from a third-party database to the client device. For music, reporting of entry and end points into and out of sections of complete audio tracks can coincide with musically seamless audio transitions between sections.