Round tripping as done in FCP7 doesn't really exist in FCPX but you can use XML to get to and from various audio apps.
Logic supports FCPXML so you can get your audio there without too much trouble, although I think the X2Pro application probably does a better job of organising things before import. X2Pro will also let you round trip to Pro Tools (which is its real purpose). Xto7 translates FCPXML to the FCP7 flavour and so can be handy for getting your audio into Audition or Soundtrack Pro.
Or in the case of basic stuff like a hum, you could try FCPX's built in tools.
A quick visit to Google should find you any number of good tutorials on round tripping on to your audio app of choice and a recent episode of the FCPX Grill podcast talked specifically about getting to logic and back.
You might consider plugins like Izotope RX which is brilliant or even something simple like SoundSoap which I use a lot for basic noise cleanup.