For screencasts, we use Quicktime and Soundflower with Apple OSX.
- Download Soundflower and install, thanks to Cycling74
- On OSX, open System Preferences > Sound
- For Output, select Soundflower (2ch)
- Open Quicktime Player (v.10)
- From the File menu, select New Screen Recording
- From the down-arrow menu next to the record button, change the microphone to Soundflower (2ch), but keep the volume at the lowest option
- Press record to begin; the file saves to your default location
- Check the file by turning your audio settings back to normal, not Soundflower, and you'll likely have the file you wanted.
(Thanks to California Matt on Apple.com Discussions for introducing us to some of these steps.)