I would say that "kiabálni" is more of a human activity. I can speak in a low voice, I can speak louder, even louder, very loud, and I can "kiabálni". And "kiabálni" is kind of a more articulate activity. To speak in a very loud voice. It can be completely neutral, just because of the distance or noise. But it can also be emotional, in an argument.
"Üvölteni" is what some animals do, and it came back into human use as a metaphor. "Üvölt, mint a sakál" - "howls like a jackal". (I see "howl" was found to be the definition below). My first thought was "roar" for "üvölteni". Maybe that is also an acceptable synonym. "Üvölteni" can be inarticulate, in a rage. It is more frequently emotional, angry, or very enthusiastic.
Let's look at a crowd: "tömeg".
A tömeg üvölt. - The crowd .... shouts/yells/howls/roars?
People in the crowd:
Az emberek a tömegben kiabálnak/üvöltenek. - shouting/yelling/howling?
If it tends to be just a loud noise to my ears, I would rather say "üvöltenek".
If I can make out what they are saying, I would rather say "kiabálnak".
But this is definitely not a rule, just my feelings on the topic.
"Why are you talking so loud?" - has the meaning of "Miért kiabálsz?"
"Why are you making this loud noise?" - has the meaning of "Miért üvöltesz?"