Sometimes when recording a screencast, it is helpful or necessary to record sound coming from the computer. For instance, a professor wanted to record a spectrogram in Audacity for his music and math class. It is important to record both the video and audio of the spectrogram at the exact same time or it becomes meaningless because you need to see where the playhead is at to understand the visual representation of the music. Another example would be recording a video of a person talking - the audio and the video need to be recorded at the same time because the speaking being off a little bit can be very distracting.
The problem is that Camtasia for Mac 2 does not do this well, if at all. You have to install an application called Soundflower and then choose Soundflower as the output option from what is being recorded - which we did in Audacity - but it still didn't work. The help documentation on this was not helpful at all.
So, we tried a different program in lieu of Camtasia for Mac, Screenflow, which worked great! It did need to install an additional program to record system audio, but when we did that it was fine. Unfortunately Screenflow is $99, which is actually the same price as Camtasia for Mac 2, but we already purchased licenses for Camtasia at a discount.
I wish I would have explored Screenflow back when we purchased licenses, but I made the mistake of just assuming Camtasia was a good product all around because the Windows version is excellent.