There is not much you can do in Camtasia to achieve the effect. The audio tools are to limited.
I personally would export the audio out from Camtasia and run it through a program like Audacity which would let you create the distortion easily and in many different ways to get the effect you actual want.
With your audio on the Audacity timeline you just apply one of the distortion effects or make your own custom effect.
Export your edited audio out of Audacity as a .wav file and import it back into Camtasia.
If you are not familiar with Audacity and would like to try going this route let me know and I'll put together and quick run through how to do it.