Can I take the soundtrac from a PP file into AU and improve the sound?

I'm afraid that camera mic audio is invariably awful, and you can't really make the actual signal any better, whatever you do. The only things about it you can (sometimes) alter aren't directly related to the signal - like background noise, and sometimes you can improve an uneven frequency response. But the signal itself is inevitably distorted - and that's the one thing that cannot be fixed, because there is no 'correct' reference for it.

So you might be able to improve it a bit, but there is no replacement for recording your source from rather closer, I'm afraid. Taking the audio only from Premiere is relatively easy - just highlight the track and select 'Edit in Audition'. When you've finished, save the file and close Audition, and your alterations will be rendered back to Premiere.