Score-Based Interactive Music Systems are involved in live performances with human musicians, reacting in realtime to audio signals and asynchronous incoming events according to a pre-specified timed scenario called mixed score. Building such a system is a difficult challenge implying strong require- ments of reliability and robustness to unforeseen errors in input. We present the application to an automatic accompaniment system of formal methods for conformance testing of critical embedded systems. Our approach is fully automatic and based on formal models constructed directly from mixed scores, specifying the behavior expected from the system when playing with musicians. It has been applied to real mixed scores and the results obtained have permitted to identify bugs in the tested system.