It seems like any scientific understanding of music is incomplete without understanding singing in gibbons: http://www.gibbons.de/main/sound.html

Specific reasons for gibbon song rather than others:

1) duh, they are apes

2) unlike birds and cetaceans, but like humans, gibbons try to aim for a pure tone when singing, with energy concentrated at the fundamental frequency

3) unlike birds and cetaceans, but like humans, gibbons try to harmonize when singing in groups

This article gets it wrong from the start... "But whereas speech is most obviously important for communicating propositions or concepts, obtaining such knowledge, this is not the primary function of music." There's the talking drums of Native Americans, rap music, and so much more.