Fadeout BGS tells the system to start fading it out right then and there, not wait 30 seconds then fade it out. If you want it to only play for exactly 30 seconds you would have to do
Wait(1800) frames (you can't do 1800 in one command, so split this up as needed)
Fadeout BGS 1 second.
Though...be advised that all attempts to time it for exactly 30 seconds is very dependent on how fast the game runs. If the game runs at say 30 FPS instead of 60 FPS, what I posted here will have it wait 60 seconds and not 30. However, there is no way around that (that I know of) short of make sure your game can run at the default speed of 60 FPS.
Use what I posted as a parallel process when the race starts (make sure it is triggered by a switch, which you turn off once the race is over). It will work then. Otherwise, even if you get a 30 second fadeout working, the last 15 seconds or so are going to be unhearable, due to how faint they are.