How can the program determine the length of MP3 files?

By simple calculation. This information is normally not to be found within the file. The formula for calculation that applies is: "length in seconds=data size in bits divided by bit rate in Hz". In fact it is a little more complicated than that, because the length of additional information (for instance IP3 TAGs) has to be subtracted from the file length. MP3PlayBase takes this into account. However, there is a case where all this does not work: Some MP3 files might have a variable bit rate (VBR). The length MP3PlayBase determines for VBR's might not be correct... We're working on it... ;-)