Computer-based methods for time-based media composition of programs that include secondary material. The methods involve searching for secondary media available from secondary material providers over a wide-area network, selecting secondary material, receiving metadata pertaining to the selected secondary material, inserting a representation of the selected secondary media into a media composition, playing back a sequence of the media composition that includes the secondary media, wherein playback of the secondary media involves playing an evaluation version of the secondary media streamed to a media composition system over the wide area network, and displaying metadata pertaining to the selected secondary material. The media composition system facilitates purchasing and transfer, and re-linking of the secondary media for inclusion within the media composition. In-context previewing of secondary media facilitates selection of the optimal secondary material, accelerated workflows, and improved media cost control.