low voice in the background downsize

Depends. If it's a stereo recording and the main voice is central, with the other voice off to one side, you will be able to alter the balance with the Center Channel Extractor. If it's mono, you don't really stand any chance at all unless you gate the quiet voice - and that generally sounds weird, because it will still cut through when the louder voice is speaking. There really aren't any other options within Audition, I'm afraid.