So I'm starting to dabble in recording a bit with GarageBand. Basically what I'm doing is dragging in a track from my iTunes collection, then recording myself playing guitar to it. After I've recorded it, I play it back, and my guitar recording is off beat by about half a second (more than enough to be noticeable).

I then turn the volume down on the original track, record myself sining to the guitar track I recorded, and when I play that back, my voice track is about half a second off beat from my guitar track.

Is there a setting or something that I'm missing? Why are my recordings out of sync with each other?

I don't know the answer to this. I keep planning on using GB to record stuff and never get around to teaching myself.

I do know this much - the external interface you use to connect your mic/instrument can cause a lag.