I'll have more to say on this (probably using Frank Miller's "Daredevil" as an example) but here are the basics: comics use visual cues – like size and shape of balloons and size and shape of letters – to reveal the qualities of sound – like how loud it is. Generally speaking, the fatter and bigger the letters, the louder the sound. We already know that "Sarge" in Mort Walker's "Beetle Bailey" is a loud man; let's watch him prove it:
Note how not all the letters in Sarge's yelling are visible. Why not? Because they're so big that there isn't even room for them in the panel! Ergo, they have to be very loud, don't they?
…Or do I mean VERY LOUD?!!