Hey guys, I think I solved this. I have the same problem. Im running FCPX 10.0.9, latest version of Lion, and I ran into this problem. The issue stems with what file you are trying to copy FROM. The root cause seems to be related to a bug in the code related to the audio PAN attributes. If the file you are trying to copy from has a PAN mode set to anything other than "NONE", then you will NOT be able to copy/past attributes from the source file with the custom PAN mode to a destination file whose attributes you want to modify.
To solve, I simply reset my source file's PAN attribute to flat, and i removed any added "denoisers" (which sidenote I don't think helped, because prior to the problem, I have been copying/pasting denoisers for some time).
Enjoy!
EDIT: I just retried it without removing denoiser. All you have to do is set the PAN mode to "none", and the problem is fixed. CHEERS!
eversic
p.s. 13 years in Information Technology Support sometimes comes in handy. 😉