How can I record a video using my webcam?

How do you record an instrument and an mp3 track simultaneously using a webcam for the purpose of posting a cover video to YouTube - all I get is the instrument even though I hear the mp3 during rec?

• Answer:

  ------ Follow these careful instructions: go to system preferences, click on sound, press the backspace button, press speech, change your system voice to princess, save preferences, exit out of system preferences, open itunes, click on any song, pause it for the duration, open a new garage band project, (any other program like this works as well), press record on garage band, go back to the itunes window quickly, press play on the desired song, sing along if that is what you want to do, change the voice level to soprano, as any other level will not be heard easily on youtube, (don't worry, it will sound normal), save the project into any folder, go to youtube, add a new video, upload the video onto the file, but don't press "OK" yet. Then open up a new tab in safari/chrome/firefox/internet explorer, go to youtube, log out, then in again to your account, (this helps refresh and speed up the download), type in "never gonna give you up" by Rick Astley, press play to listen and...