In that case I would not use merge, and probably not mix either, but combine or blend would be fine. I would use combine if you are saying what the composer did, but blend if you are saying what is being done by the singers and/or conductor, i.e. adjusting the balance to achieve an optimal result. You can even use balance as a verb. It is probably also better to say harmonically than musically, if there would otherwise be any possibility of misinterpreting your meaning.