I always assumed this game was going to be super speedy, and yet the rocks move ever so slowly compared to the foreground (the parallax scrolling is pretty neat by the way). You should probably widen the screen to some sort of super widescreen ratio (3:1 or 5:1 ?) and increase the rocks' speed. I'm not too sure about the music either, it's too mellow for a screaming exploding guy (where is that scream by the way?).
That's my 2 cents anyway, sorry if you weren't looking for criticism.
Your speed increases as the level increases (level display is on the bottom left). That's one of the on screen items we want to bring more attention to. While it may not look it, even at these early levels and at this slow speed, the game is fairly tough, I have yet to make it past level 10, which is long before the wall of fire and falling rocks start to appear.
While a scream would be novel, it could also get pretty damn annoying for a lot of people. So i chose not to include it.
Quite a few people have commented on the music, I'm considering changing it. Perhaps.
Rocks! We can see them now.