Is will most likely not improve the sound, but at least it looks quite cool. This little 'build log' video on Facebook outlines how you might go about re-shelling your beloved earphones with two spent bullet casings.
This handy 'guide' goes through the process step-by-step, showing how you can use simple household tools in order to complete the job. With the tool list including things like a drill, hacksaw and pieces of sandpaper with various grit ratings, doing it yourself shouldn't really take more than an hour.
If you're interested in doing this yourself, watch the video posted by Best Knives above.