How do I record Sound from my computer

Some audio interfaces allow this. My RME interface has a "loopback" facility which allows any hardware output to send its signal back as an input for software to use. Nothing needs to be attached to the hardware output, all the routing is done internally so the signal never leaves the digital domain.

If it's going to be possible at all, you'd have to set it up as a source in the Sounds control panel, but not all systems allow this, thanks to the RIAA and their copyright paranoia... it depends upon who made your computer. The approach I use involves an app called SoundTap, which does the job outside of Audition (and bypasses the record settings to get at the incoming stream directly), and it works fine.