Z.B. in einer MP3 stehen ja nicht nur die reinen Musikdaten, da kann man noch ganz andere Infos hinterlegen. Daher dürfte BASS_ChannelGetLength exakter sein.
Zitat:
Remarks
The exact length of a stream will be returned once the whole file has been streamed, but until then it is not always possible to 100% accurately estimate the length. The length is always exact for MP3/MP2/MP1 files when the BASS_STREAM_PRESCAN flag is used in the BASS_StreamCreateFile call, otherwise it is an (usually accurate) estimation based on the file size. The length returned for OGG files will usually be exact (assuming the file is not corrupt), but when streaming from the internet (or "buffered" user file), it can be a very rough estimation until the whole file has been downloaded.