This combination has definitely given us issues before. The reason is because the C920, for some reason, only supports sampling rates of 16k, 24k, and 32k. In order to guarantee that your audio stays in sync, the input and output sample rates must be the same and unfortunately, Apple's built in audio output only supports 44.1k, 48k, 88.2k, and 96k.
The best solution is to use a separate microphone that supports a higher sample rate. The audio quality on the C920 is quite poor.
That said, I was able to find a workaround. In that Audio & Midi Setup application, click the in the lower left to create an Aggregate device. You can name it Character Animator. Choose the C920 as your audio input and the Build-In Output for the output. Choose a sample rate of 32k(you might need to switch the C920 to 32k ahead of time for this to work).
This will force the built in output to resample to 32k so it will work in our application. Please post back if you have any issues getting this to work.
This did not work. Character Animator still states that the sample rate is not supported.
Sorry Susy - if Dan R's workaround didn't work, then it doesn't seem like we have a current solution. Your best bet is probably to buy a cheap USB microphone (searching on Amazon I found many under $10 so it shouldn't be too bad).
We will continue to look into this and see if we can figure out a solution to keep the audio in sync for lower sample rates in the future.
I've since reverified that creating an aggregate device with the Logitech and built in audio works, but admittedly, it's not an ideal solution.