I don't know what Barks or Rosa may have adddressed about Donald's voice.
When reading comic books I more or less assume that all characters are speaking with similar voices. And when I read Bark's comics I assumed that Donald didn't speak any differently than any character, but I didn't wonder much about that difference from the cartoon Donald.
The creators of the Duck Tales 2017 series are fans not only of the Duck Tales 1987 series but of the comic books by Barks and Rosa and others.
Donald in Duck Tales 2017 almost always speaks in a "Donald Duck" type voice which is hard to understand. If I remember correctly, in "The Shadow War", August 18. 2018, Huey, Dewey, and Louie discuss how much trouble they have trying to understand what Donald is saying. In one scene Gyro Gearloose makes Donald swallow a tiny device which makes Donald speak clearly - with a differnt voice actor. At one point Mrs. Beakley tells Donald to head for the storm, Donald says "I am the storm", and Mrs. Beakley asks if he has always been saying things like that.
The device is destroyed during the final battle and Donald reverts to his normal voice.
Gyro calls the device a "Barksian modulator", showing that the creators of Duck Tales 2017 believed that Donald had a much different voice in the comics by Barks and others than in the cartoons and movies.
And of course this is not information from Carl Barks or Don Rosa themselves, but what other fans of their comic books think about Donald's voice(s).
And perhaps you could see if Don Rosa has a website or blog and if it talks about Donald's voice. Maybeyou could ask about it.