How to use Adobe Audition to record outgoing Google voice calls on my PC?

A second vote for Virtual Audio Cable.

Otherwise you could route the audio through an audio interface that allows recording of computer audio.

Chances are that you aren't going to find these sources accessible to Audition either - they do that on purpose. There are two things you can try though. The first is, if you can hear both sides of the conversation, to try using something like the SoundTap streaming audio app - others may be available but I know personally that this one works pretty well. The other alternative, which in theory will let you stream both sides of the call to Audition is way more complicated to set up - and that's to use Virtual Audio Cable. The difficulty with it is that it's deeply technical - it has to be, because it's extracting streams that you aren't supposed to have access to...