If you downloaded fg-optional-bonus-soundtracks.bin alongside FlightGear-version.exe and fg-data.bin :
If you love the game’s music and want to own the tracks offline, download fg-optional-bonus-soundtracks.bin fg-optional-bonus-soundtracks.bin
Yes, in most cases, you can delete this file. Since it is marked "optional," deleting it will likely only mean you cannot access the bonus soundtrack within the game. If you downloaded fg-optional-bonus-soundtracks