So, Smule says:
This free edition is designed for those cheap people unwilling to cough up a buck for the paid version.
Well, regardless of the reason, the price is right (free) for what looks like an amusing virtual musical instrument for your iPhone.
Leaf Trombone: Lite & Free (iTunes App Store)
There is only one default track for the game play in the free version. However, you can still compose and play your own songs on it just like the paid version.
YouTube video courtesy of Smule