Every aspect of biology serves an evolutionary purpose at some level. It has all been selected for at some point in time. Many behaviours are innate, including many (most?) of our own. However, the fact that we have many innate aspects to our own behaviour does not affect our functioning as conscious, aware, individuals with agency (or does it?). It is quite a stretch to presuppose that all the other species lack consciousness and agency, which is the default assumption for much research. I believe that assumption is probably wrong. Evolution has resulted in us having very well developed cognitive abilities, but we are not the only species with a brain, and most of the rest have broadly the same capabilities which we do. What separates us from the rest is advanced spoken language and written language, and even then, there are plenty of examples of other species which can communicate with their own basic language, or can learn ours. Our cognitive abilities might not be that much better than many other species; the differentiator is advanced language.

The general behaviours observed in a species does not imply a lack of conscious will governing individual actions. It's certainly possible that vocalisations could be involuntary. However, given our understanding of evolutionary and developmental biology, I think that it more likely that the opposite is true. However, disproving this hypothesis definitively would be quite difficult. It's difficult enough for individuals within our own species!