Retrieval systems for polyphonic music rely on the automatic estimation of similarity between two musical pieces. In the case of symbolic music, existing systems either consider a monophonic reduction based on melody or propose algorithms with high complexity. In this paper, we propose a new approach. Musical pieces are represented as a sequence of chords which are estimated from groups of notes sounding at the same time. A root and a mode are associated to each chord. Local alignment is then applied for estimating a similarity score between these sequences. Experiments performed on MIDI files collected on the Internet show that the system proposed allows the retrieval of different versions of the same song.