In this paper, we consider the problem of underdetermined blind source separation using modal decomposition. Indeed, audio signals and, in particular, musical signals can be well approximated by a sum of damped sinusoidal (modal) components. Based on this representation, we propose a two steps approach consisting of a signal analysis (extraction of the modal components) followed by a signal synthesis (pairing of the components belonging to the same source) using vector clustering. Our contributions in this paper are a new separation method with relaxed assumption and reduced computational cost compared to other existing algorithms. Simulation results are given to assess the performance of the proposed algorithm.