Garage Band

ok, you can use any loop in any sound format, but stick with .aif(f) or .mp3.
What you want to do, is just drag it on to the timeline, creating another layer. Move your mouse on the upper half of the end of the sample, and drag to the right while holding down the mouse button, this automattically loops the sample. Now whatever tempo you set the song to be at, all other sample's will be configured to that BPM.

Hope that helps. Also, go to Apple.com, and download the .pdf manuals on how to use the program properly.

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