AIFFs and OMF's

If exporting from Final Cut to Soundtrack

and you just want a portion of the audio:

-make in and out points and turn on the tracks (two at a time) that you want. AIFF format can be read by any audio editing software but all clips on a track get consolidated

-if you are ready to work with the whole audio track (presumably after everything is in place/in sync in the FCP timeline):

then more flexible, and appropriate for Soundtrack is either use the "Export/for Soundtrack" function which will give you a reference movie, or export as OMF. The only downside to the OMF is it will export your ENTIRE soundtrack. But it will preserve the various tracks that you have an allow you to save them with "handles" (a few seconds of extra sound on either side of your clips) which are useful.

Export out of Soundtrack and back to Final Cut Pro as AIFF files. 16 bit. 48kHz.