If you have more context about where this file came from, reply with details for a targeted follow-up.
If you have already downloaded it, the setup.exe will detect the file. If you decide you don't want it, you can usually uncheck "Bonus Soundtrack" in the installer options to save installation time. fgoptionalbonussoundtracksbin
If you know the offset and size of the track: If you have more context about where this
In short, it is a compressed archive containing the game’s music, separated from the core game files to save download bandwidth. Why is it Separated? fgoptionalbonussoundtracksbin