The post is in German, but the pseudo code is English.
Windows Media Player 10 calculates the duration of the file based on the Xing header. VBR files encoded by Fraunhofer encoders will not work correctly with WMP (what the duration is concerned) because they use a VBRI header.
Older versions of WMP used to read the duration from the TLEN frame of ID3v2.3 tags.