You could probably find some music on Newgrounds. Since all the music there is Creative Commons you are free to use it as long as you credit the person.
Ah well in that case, regarding the loading screen, I would suggest moving the controls to somewhere near the top, as most people won't start reading at the bottom, or even realize the controls are down there till they reach that point.
I had a lot of fun playing this with my friends. Personally I think the difficulty of getting through the gaps is just right. After a few games most of us were getting the hang of getting through them. What you want to do is make sure you don't come in at too much of an angle and aim for the larger gaps and you should be good.
Anyways I have one suggestion, replace the default music with some music like what you have in the trailer. I was disappointed to not hear any when I played, really helps to set the mood.
I don't know if this is a problem with our connections or the map. But every once in a while we would all get a lot of lag for a single random round of the game. Which is more of a problem in this sort of game. I don't know if you have any idea what might cause that, but something to keep in mind.
If you wanted you could also make the loading screen require you to press a key to continue. The game loads far to quickly for people to read usually.