Abstract. In this paper, we describe a CBR system for applying musically acceptable tempo transformations to monophonic audio recordings of musical performances. Within the tempo transformation process, the expressivity of the performance is adjusted in such a way that the result sounds natural for the new tempo. A case base of previously performed melodies is used to infer the appropriate expressivity. Tempo transformation is one of the audio post-processing tasks manually done in audiolabs. Automatizing this process may, therefore, be of industrial interest.