The process of changing regular speech bubbles to more distinct ones has been common enough among comics since at least the mid-silver age. At the time it took more of the form of the speech bubble itself, with different shapes perhaps indicating a different mode of speech compared to the remainder. For instance, a robot character might have a square shaped box to indicate a more technical approach to speech (geometry = mathematics = robotic language?) even if the written words themselves did not appear to be different. As the medium has evolved, the letterers have done more with speech bubbles, giving the written words of character like Thor, Etrigan or the Spectre a distinctive style where the reader can use visual clues to evoke the sound of the voice. Recently, I read the Bluewater biography of Stephen Hawking (which was pretty good) but one thing which I found frustrating was the use of the voice bubbles in this case:

In this case the robotic speech like aspect of the bubbles is indicative of his disability where he needs to talk through a computer. In this case it is a difference of outlook in what is highlighted in a character. In the case of the other comic characters it is a distinctive nature of their powers which gives them a separate voice pattern, but in the case of Stephen Hawking it is as a result of his disability. In this case I am not sure if this specific nature of the person needs to be highlighted in this way. Most people who would read this comic would already familiar with the man and the laboured nature of his speech. They even reference within the story how difficult it is for him to speak in this way, yet the speech bubble gives it a false impression for a number of reasons.