1 - Download and install Soundflower -
This is a great little item, free and really works well with no problems. I've used it for a variety of purposes for several years. Essentially it is a software simulation of a sound input or output - it lets you connect the "speakers" of your computer to software that you wish to direct sound to. In this case - QuickTime Player. It installs itself in your system and when completed it will reveal itself in your System Preferences/Sound
There it is! You now see Soundflower as an Output source, and if you peek at Input you'll see it as an option there.
2 - Set Soundflower as your Output Destination
Refer to the screen shot above. It shows Internal Speakers as the Sound Output destination. That's our normal way to process sounds from our computer - we click a video and hear the sound through our speakers or headphones. When we set Soundflower as the output, we re-route that sound to software - which QuickTime (or other recording software) can utilize for our screen movie. If you try playing sounds now you won't hear anything - don't freak out, you told it to divert the sound to the software, remember? Just click back to Internal Speakers when you are done.
3 - Set QuickTime Player to use Soundflower as a sound input
Launch QuickTime Player, and go to the File Menu and select New Screen Recording. You'll see a control like this:
Click the triangle on the right side, and you'll see this menu:
Select Soundflower as you see above. Note the ability to Select Built in Input or Internal Microphone, if you just want narration, this allows you to use an external audio source or your mic to do just that. If all you want is narration, just selecting Built-in Microphone will work just fine. Handy for a how to screen movie!
Click the red record button on the Screen Recording control to start your movie, Start your video or music background source and record away!
Click the square stop recording button where the red start record button icon was and you'll be presented with your brand new screen recording including audio! You can click the Play button to preview your creation.. but wait - you just clicked play and don't hear anything! Yikes! No prob... remember how you went to System Preferences/Sound to change your sound output to Soundflower? Its still set there, so you can't hear it! Set it back to Internal Speakers, go back to your new movie and try again! Bada boom! It worked....
6 - Save and share!
So you can either go to File/Share and select your favorite web video host, or click the red dot on the top left of the QuickTime movie window which will prompt you to name and choose a local destination for your creation! The output of this process is kinda low-rez for video as it has a low framerate an may "flicker" a bit, but it works very well for a how-to screen capture or to get the gist of a video recording. This is great for making a podcast or a how-to video to share with staff members. Hope this helps and saves you some bucks!
If you want to get real fancy, this amazing blogpost explains how to have sound background AND narration. I've tried it and it works, but its a teensy bit complex. Check this out if you want to go to the next level! Major geek cred if you can do it!