What design factors contribute to an illusion of agency in a computational system? Our previous research [1, 2] has investigated this question in the context of creative human- machine musical partnerships, where we identified musical behaviours implying machine agency from the perspective of a human performer. This paper investigates an audience perspective: what factors contribute to an impression of machine agency for in a musical performance? Audience feedback data was collected during a concert with four performances, each comprising a human musician interacting with a computer music system. Three performances utilized a computational agent, CIM [1], designed for this research. The fourth performance utilized an array of effect pedals designed in an extended instrument paradigm [3]. The audience feedback questionnaire queried whether a sense of machine agency was imparted, and to what degree visual, spatial, timbral and musical factors contributed to this impression. The results showed our CIM system succeeded in imparting a sense of agency, and that all four of the suggested factors contributed to that impression.