A number of models for integration of retrieval of multiple types of multimedia data have been proposed in the past. These models typically impose a certain unified representation model and/or indexing methods on all media types. This paper presents a work in progress on design of a multimedia information retrieval system with extensibility providing integration of existing multimedia retrieval software modules and choice among different retrieval models. The system manages multiple classes of documents, with each class being defined by an XML Schema document. The documents themselves are stored in XML format according to the given schema. Design of the main subsystems — document storage, indexing, and retrieval — is presented, as well as an example of a retrieval model.