As XDA website’s blocked & google wasn’t much use, thought i’d ask the STW massive instead…

Does anyone know why for some MP3s I can’t skip forward or backwards to a point in a track using the slide-bar? It just goes back to the beginning when I try, but doesn’t do it for all similar files.
They’re long 2hr MP3s, if that makes a difference?
It happens with both the default music player and Doubletwist app. and I noticed that the length is shown as a negative figure for some of these files, which is odd too!