This paper describes an interoperable metadata model, based on international standards, which has been designed to enable the management, description, tracking, exchange and sharing of multimedia resources both within and between cultural institutions. Metadata standards have been developed to describe museum objects (CIDOC Conceptual Reference Model) and to describe multimedia content (MPEG-7) but no standard currently exists for describing museum multimedia content. The Harmony project's ABC model aims to provide a core underlying model to enable a common understanding of metadata terms across disparate domains. This paper describes the ABC ontology and how it has been used to facilitate the merging of the MPEG-7 and CIDOC-CRM models into a single ontology for describing and managing multimedia in museums. The resulting model is demonstrated and evaluated by applying it to a number of typical examples of multimedia museum content. The outcome is an extensible model/standard for describing museum multimedia content which could enable the open exchange and sharing of such resources and a common search interface across multimedia collections distributed across cultural institutions.