This paper starts from an experimental interactive musical installation designed at IRCAM: “LISTEN LISBOA”. This installation relies on a perceptual paradox: spectators are walking into a real space, see this space and at the same time hears through headphones a virtual sound space, mapped to the real one. In the first part of this paper we present this installation and more precisely the interactive musical “scenario” designed by the composer. We derive from this example general need for tools, used by composers working on this kind of installation. We show that these needs are close to those of video game level designers and associated scene languages with some extensions.