In the examination of the local sound environment, the majority of studies have been biased towards artificial aspects such as the control of traffic and machinery noises, design of comfortable sounds in living and work space, and soundscape design based on natural, cultural, social and historical factors in the locality. While these studies have certainly contributed important outcomes towards our understanding of the sound environment, discussion on natural sound elements tends to lack biological foundation. The present paper introduces the question of how we can evaluate natural sounds in terms of the ecological basis and biological diversity which are the central concern for conservation of the natural environment. We experience the natural sound environment as auditory phenomena through hearing natural sounds, both biological and physical, in acoustic dimensions of a specific medium and particular space. Although what we hear is changeable in time, quantity and quality as intrinsic features of natural sounds, it does not mean that natural sounds are unreliable measures for the biological diversity of the local ecosystem. However, we need optimum approaches in sampling and evaluating sound sources. In the study of the sound environment of both natural forest and urban parks in central Japan, examination of species number, composition and frequency showed that 6-minute sample recordings of the local sound environment cover over 90 percent of natural sound sources, suggesting the optimum sampling time. From the comparative study of woodland and pondside habitats, it was clear that differences in the characteristics of a sound environment can be monitored by surveying the temporal and spatial distribution of sound sources. Further, natural sound diversity can be related to the biological diversity of the local ecosystem by evaluating sound sources in terms of taxonomy, locality bond and trophic levels. In naturally enriched areas, animal sound sources are high in number and taxonomic diversity. As for avian sounds their sound sources are well-balanced in trophic levels and composed of locally characteristic species. Natural sound diversity can be used for valuable environmental indices by reference to taxonomical and ecological aspects.