Sound content description is one of the aims of the MPEG-7 initiative. Although MPEG-7 focuses on indexing and retrieval of audio, there are other sound content-based processing applications waiting to be developed once we have a robust set of descriptors and structures for putting them into relation and for expressing semantic concerns about sound. Spectral Modeling techniques provide a valuable framework for extracting and organizing sound content descriptions. All our descriptors can be considered low- or mid-level, for which we have devised automatic extraction procedures. In this paper we present the basic descriptors and introduce a scheme for organizing them (a so called "Descriptor Scheme"). We do not cover the high level description of music (musical forms and styles, roles of characters in a movie, etc.) which is also relevant in MPEG-7.