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