It's not bad at all for one of your first games. The jumping and movement are pretty good, but perhaps you can add more than one sound. The gravity change is interesting; perhaps you can add some arrows to indicate the direction as well. I made it to the part were you need to quickly move to the left and right to avoid the red spikes. That part is just too difficult, you basically need to start moving in the right direction before even knowing what that direction is.