QuickTime Screen + Audio Capture
This is how I did it. You may need a pair of headphones if you use the bult-in microphone.

Open quicktime and click File->New Screen Recording 
Select the audio source by clicking on the small down-pointing arrow 
Turn up the sound that appears below the record button 
Click File->New Movie Recoring 
Resize the movie window and place it where you like it 
Click record on the screen recorder window