Mavericks QuickTime player not recording computer audio
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.