I found it worthwhile to go to the original article and listen to the sound samples of the calls. The contrastive element ‘oo’ is a very low sound, which I find barely perceptible. But if you look at the spectrographs of the recordings, you can see that the additional ‘oo’ element is there, producing a dark band at the low frequencies and warping the formants of the previous call sounds. These calls also have about twice the duration of the single-morpheme call.

I’m a bit surprised that the longer, more complex calls have a more general meaning, and the simplex calls a more specific one. So ‘hok’ meaning eagle but ‘hok-oo’ meaning general canopy disturbance. Or ‘krak’ meaning leopard but ‘krak-oo’ meaning general alert. This seems the reverse of what you commonly find in human language where the simpler expression is often the most general, for example ‘truck’ compared with the more complex and semantically specific ‘Ford truck’.