Comics are visual, but so much of their meaning comes from the sound they imply. Be it direct dialogue or narration, most panels in a comic are full of implied sound. Like this example from Dick Tracy.
The most interesting panel in this is the final one. The 'QLUNQ' really makes me feel the sound of a briefcase full of cash hitting a skull. There is actually a word for these kind of words. onomatopoeia
Used in the right way, onomatopoeia can convey more than the image--even with movement lines--could ever achieve.
Each genre of comic has their own conventions of these sound-words.
Old action comics have the classics like 'POW!' and 'THUMP!' made famous by the kitsch 'Batman' tv show, and even artists like Roy Lichtenstein who made high art out of the comics he read as a boy.
But sometimes D.C was a little too literal. (not sure a fist exploding a face makes that sound)
Marvel was a little more creative:
But even they often went too far....
Manga has a similar style.
As did Asterix
The point is, you have to match the sound you imagine to the words you read. Of course, given this is a visual art, you also get to write the word in a style that best conveys its sound.
Visualizing words this way is great, but when it comes to character voice, it would take over the page to have every speech bubble filled with exploding bold lettering.
In this Wolverine example, you can see everything coming together. We have the onomatopoeias of 'SNIKT' and 'KRRIIIPP!' and Wolverine's dialogue in the last panel is divided between two speech bubbles, the second clearly conveying that he is yelling.
Part 2. In which sounds are made.
Step 1. Listen to the following sounds and come up with a visual onomatopoeia for each one.
Now, on a new page in your folio, give yourself 3 empty panels.
Your job is to make a Three panel strip of a character who is surprised by one of your onomatopoeias in the first image, reacts to it in panel 2, and then creates a sound of their own in the final image.
If your character needs to speak, play with some of these speech balloons to give them a voice.
Well get going......
Well get going......