If you can tell the difference between the aggressive and play barks of your dog, you are to be congratulated. A study of the ability of humans to discriminate between these categories of barks found that the humans could not tell them apart, even though the dogs could tell the difference (Molnar et al. 2006. Behavioural Processes 73: 76-83).
And a study of dog barks found that there were structural acoustic differences between barks produced in different contexts (Yin & McCowan 2004. Anim. Behav. 68: 343-335), just like we have structural acoustic differences associated with our words.
There might indeed be emotional encoding as well as a meaningful message in the barks, but how is that different from saying “I am angry” in a neutral tone of voice, vs. saying “I am angry” in a voice that is shaking with rage? My point is that many people make assumptions and then make categorical statement about the absence of meaning based on those assumptions (by the way, I approve of the ... There does not seem to be.... showing a willingness to consider other possibilities).
Some animal vocalizations are analagous to tonal languages, such as Chinese or Navajo, where small differences in tones or sound frequencies can change the meaning of a word. These tones can be assembled into a short vocalization that might sound like a simple “chirp” to us, but when we analyze the acoustic structure, we find a considerable amount of complexity. For example, prairie dog alarm calls sound like the chirps of a bird, but when we analyze the structure and perform experiments to tease apart the different components of meaning, we find that a single chirp might encode “ tall thin human in yellow shirt.” (Disclaimer: all we know is that the prairie dogs have consistent sounds in their alarm calls that are associated with “tall” “thin” “human” and “yellow shirt” and we get a different set of sounds associated with “tall” “thin” “human” and “blue shirt”, where the “tall” “thin” “human” sounds are the same, but the “blue shirt” sound is different from the “yellow shirt” sound. We don’t know what sort of mental concepts prairie dogs have of any of this).