The usual is that the bit rate is not exact and will be off a little. Usually not enough except for every second counts. Since you are an old time user that's what I'd check out.
I imported an mp3 file into MM without any problem (I've done it many of times before).
When I check the file's properties it's truncated by almost a minute (122 seconds vs 28 seconds).
If I play the same file in Media Player it runs the full time.
I have NO idea what's going on.
I'd appreciate any help/suggestions.