Sound in a purely visual medium is problematic.
I generally avoid it. Type and emanata are the only tools available, and both of them tend to compete with the visual story rather than compliment it.
The roundness of a word balloon, for instance, is hard to manage inside of a square panel. The contrast between straight and curve is fierce, and (unless artfully placed) distracts from the image and feels like clutter. All emanata is inherently foreign to the drawing. And the more representational the drawing, the more out of place the emanata or type. Symbol fights representation.
This is how I deal:
I treat elements within the panel in much the same way I approach the panels within a page: I subdivide. A round balloon is a foreign object, but a squared balloon is a subdivision. It's clean. If I quarantine my emanata with a square balloon, I can keep it from fighting with my image. I like a clean read, and with so much happening on any given page of comics, I have to cheat any way I can to get it.
If I were more talented, I wouldn't have to resort to this nonsense. I'm constantly impressed with the elegant solutions I see from other artists and letterers.
Also, not everyone wants a clean read or a harmonious page. This is how I like to MAKE comics, but I personally enjoy reading intentionally frantic, carefully cluttered work when it's done well. I picked up MITCHUM by Blutch last week while I was in LA, and he doesn't even bother with panel borders for most of the book. I wouldn't even know how to approach that at this point.
Maybe one day... but not today.