I was recording the screen on my computer that I had updated to Mavericks, and when I replayed the video, there was no audio except the audio captured by my mic. The old Qucktime worked fine, but now it's not.
I was able to do this by going to Utilities>Audio Midi Setup
Click the + sign to add a new device, choose Create Aggregate Device and click both the Built-In Microphone and Built-In Output checkboxes. Then right click the Aggregate device you just created and choose. Use this for Sound Input. Now this device will appear as in input choice in QuickTime Player X.
Now open QuickTime Player X, choose New Screen Recording and click the drop down arrow. You will now see Aggregate Device as a Mic choice.
Make sure you selected an input source(i.e. your internal microphone):
EDIT: I tested it with Mac Os X 10.7.4. Like I thought, recording system audio was never implemented(assuming they did not added it in 10.8 just to remove it again with 10.9)