A BNK file might contain additional data regarding video content. In some cases you will need to first convert the file to a known video file format, then use an application to extract the audio from the file. However, the conversion between BNK and MP3 can be easily achieved using Freemake Free Video Converter. It seems that is among the few applications which has support for BNK files. You can use it for video conversion also because of its high functionality but beside that you can also convert to audio file formats.