A few things to try:
Look at the [Playback Range] tag. It should be empty for most normal files. If there's something there, it might be causing the length issue.
Try updating the file from it's internal tags with Library Tools > Update Library (from tags) . This will force MC to re-read all of the internal tags in the file, and maybe make it's length correct again.
Finally, you might look at the [Duration] tag. You cant edit it, but it should have the actual length of the file in it.
Brian.