FL studio

This is another software that similar to Garage Band. You can play with it to make your own music. Moreover, the advantages of this software is you can export the song that you create in mp3 file or in wav file. These format can easily import into your flash file or in your movie.
You can download the demo :
Download demo

Here are some tutorial step by step about how to use FL studio software with pictures to illustrate:

Basic Tutorial
Tutorial part 2
Tutorial part 3