A server system with one or more processors and memory receives, from a client device, information including one or more criteria for a media item to be generated. In some embodiments, the one or more criteria include one or more audio tracks for the media item to be generated. In some embodiments, the one or more criteria include one or more keywords for the media item to be generated. The server system identifies one or more media files in a database of media files for the media item to be generated based at least in part on the one or more criteria. The server system sends, to the client device, first information identifying the one or more media files. In some embodiments, the server system also sends, to the client device, synchronization information for synchronizing one or more audio tracks with the one or more identified media files.