If you think about it, the speech balloon is an odd convention, and this site tells how the convention evolved from banners and flags in the Renaissance to the current bubble or balloon.
Interestingly, Aztec codices contain similar shapes, called “speech scrolls,” that appear near the mouths of figures said to be speaking. Words or sounds aren’t represented in the scrolls themselves; they are simply meant to represent that someone was speaking. This convention is reiterated in contemporary Mexican design, notably in fire emergency signs, where the stick figures spreading an alarm of “Fire!” (or “Incendio!”) have speech scrolls coming from their mouths. I’ll post a picture when I can find one.