Welcome
First, if you are going to include an 8mb reward, make it more than a song. A video clip would be acceptable to me. A song with a slow intro is just a waste. If you really want to include the song, add it as music while you are playing the game.
Add more sound effects. One for hitting a green atom and a different one for hitting a red atom would be sufficient for this version. In game music would also be nice.
I can't think of any storyline ideas, but you could check out Atomik Kaos
http://www.newgrounds.com/portal/view /460353
It has an amusing little storyline to go with it. Storylines don't have to be serious to be good.