Yesterday I faced this problem.
When I imported a audio file into a movieclip of a heavy flash file, It imported the audio but it was truncated (the end portion of the audio is missing)

I even tried importing the audio into new flash file and drag and drop it in the library but still got the same problem

Finally I managed the situation by

* Recreating the movieclip in a new flash file
* Dragging and dropping it in to my project file’s library
* Swapping the old movieclip with this new one using swap option in properties window

Then it worked 🙂