option1(easy): you should be able to determine how long something lasts on your story bord, like how long a fade will be during and so....maybe if you change your songs duration into the origianl duration, it should help.
option2(hard): if 1 doesn't work, it could be that WMM can't read mp3's good, and, if i'm not mistaken you can slice things up with WMM, all you need to do is cut the track into pieces, find a lyric, then put it back together, as a *.wav for example.
if you need any help with the last option (which will not suprise me), just post here...