As of version 2.13, there is now an option to only record your game's audio (as well as any microphone that you may have selected).
Previously FBX simply recorded all of your PC's audio, whether it was from the game itself or coming from other sources such as Discord, Windows Media Player, or any other apps that you are running whilst recording.
By default, FBX will still do this but if you want to avoid having all of this background noise in your recordings, switch from System Sounds to Game Sound Only in the Audio settings.
You may notice that when you switch to your game, the FBX overlay takes a little longer to initialize than it normally would, This is because FBX quickly has to quickly figure out the format of the sound that the game is outputting and isolate it from everything else that is going on. The overlay will let you know when it is doing this and once it is ready. you will see the usual prompt to press the hotkey to start recording.
If FBX is unable to isolate the game's audio from all of the other audio or if you start recording before it has finished initializing, FBX will simply fall back to recording everything to avoid you being left with no audio at all.