At the recent meeting of the LAFCPUG (CPUG?) Micheal Wohl gave a twenty minute presentation on the advanced audio features in FCPX. Lots of good tips in there and maybe some features you've yet to discover.
Apple have already publicly announced that the next version of FCPX gets beefed up audio mixing, but there are a lot of audio features already within the app that many editors don't know are there.
Michael Wohl has written the book Final Cut Pro X Advanced Editing and from there he shares some great tips and techniques on audio. Learn how to toggle tracks on and off before and after the clip has been edited onto the timeline, analyze the audio and roll off unwanted frequencies and apply the many varied & flexible built in filters.
Many thanks to Micheal Wohl for the presentation and Michael Horton for hosting the evening.