On some level, I would posit that music is still a form of language. Music is to language what poetry is to mathematics. Mathematics (or computer programing languages) are a formal method of conveying information in a very strict structural context. Similarly, music conveys emotion in a melodic way much in how poetry conveys emotion using language. I see all these forms on a continuum of structure vs. lyrics, emotion vs. reason. But having said all this, I agree with the basic conjecture that music probably preceded language. Whether it is vestigial I am not as convinced for that, I think it still performs important functions for Human communication.