Recording while a song from itunes plays

[Soul Power]

Hey guys, I'm having trouble figuring out how to record a movie of me playing "aint no mountain high enough".
I have a 1/4 to 1/8 adapter, and i'm having no problem recording my playing in the movie but I AM having trouble figuring out how to record my playing OVER the original song...
Before I had the adapter, I would just play and record with "imovie" but the sound quality of playing using that method is terrible.
So how do I record a movie of me playing TO the track?
***Using a macbook pro, it only has one input for a 1/8 incher

[pnut166]

Two ways. The obvious one is use a mic. But the best way is to use something like Audacity to blend your recorded track with the original.

[Stilettoprefer]

^^ beat me to it

Audacity is awesome

[Soul Power]

***I got audacity but I don't know if it does what I'm asking for!
Let me try to be more clear:
I'm trying to record a movie of me playing ALONG with the song while my bass is plugged into the computer.
I don't want to just aim the speakers and my amp at the internal microphone of my computer because the quality sucks, but when I'm plugged in the quality is GREAT but I can't hear the song!
And if I CAN hear the song, it's not being recorded by Imovie, but my playing is.

[The Moose]

I think what theyre suggesting, and what I do, is to record yourself playing on camera while DI'ing the bass, then afterwards use audacity to sync up your bass playing with the original track

[bassman03]

If you have a Mac you could try uploading the song to Garageband and you should be able to just play along and record.