You can indeed use the "Levels" command to alter only a part of a clip with multiple keyframes. Use either the "Range Selection Tool" (ggg keyboard shortcut) or use IN and OUT markers and then the "Select In to Out" command from the "Mark" menu. If you select between keyframes, Final Cut Pro will include the keyframes immediately before and after your selection.
I'm cutting on FCP 7 today and maybe I'm misunderstanding the post, but neither of these options work for me. I can select a portion of the clip as outlined above, but using the "Levels" command still adjust the levels across the entire clip, not just the audio keyframes that are contained within the selection.