Comic Sound FX

Illustrated sound effects are the work of the comic book letterer.
The are visual representations of sounds in comics, often used to emphasize an action like a punch, a kick, an explosion, a blast of energy, etc.
 

Occasionally they are in-jokes for the creators, adding in the artist's name or a funny phrase.
 
Sometimes the artist can use the sound effect

as a framing device or make the sound effect see through so that it wont hinder the art in the panel, like the other example shown here.