First, the basics: bigger and bolder letters = louder sounds. It's really that simple. But, again: you need to be a skilled enough artist to make your figures look consonant with the content of their speech balloons. First panel, the angry look on the prefect's face. Third panel, the enthusiasm on the old soldier's face and his rapid hand-waving when he suddenly remembers the name of the wine dealer he gave Vercingetorix's invaluable shield to. Final panel, everybody being angry and shouting -- except Obelix, who's just shouting because everybody else is shouting; his calm expression amid the anger of the others creating humor that wouldn't be there if Uderzo weren't such a skilled artist.
Now, I was going to talk about cartoon-style comics first and then move on to superheroes, but since we looked at how big, bold letters denote louder sounds, I thought we might do just a little bit more exploration of that theme before moving on to other characteristics of sounds, like for example tone of voice.
So, let's think about this for a while... How loud can a comics sound be? What are the limits to it? I'll give a first answer here; it's actually not the correct answer, because there are actually at least two more levels of loudness, but I don't have pictures illustrating them, so I'll leave them aside for the time being.
Anyway, the panel is sort of a limit to the size of your sound effect, isn't it? So if you want to tell the reader that the sound he's "hearing" -- actually watching -- is really, really loud, in fact as loud as can be, then you can let it fill up the entire panel. But how do you do that without obscuring everything else in the panel?
Why, by letting the sound effect be the panel, of course. This example is from Frank Miller's brilliant Batman -- The Dark Knight Returns, which is warmly recommended if you haven't already read it.