In this paper, starting from a case study (the mixed-media data sonification installation Res Extensae), we discuss a number of assumptions on the efficacy of sound as a means to represent and communicate numerical data. The discussion is supported by the results of a questionnaire aimed at validating our assumptions and conducted with fifteen of the participants to the experience. At the same time, we have the ambition to contribute to a wider debate on the value of data sonification. We introduce the first stage of a research on sonification as a design-driven, user-centred and multi-modal experience, in that closer to data design practices rather than to traditional composition and computer music. We describe the usage of physical objects to help users to put sounds and data into a wider context, improving the user experience and facilitating the comprehension and retention of the meaning of data.