Read this dev workaround :
maybe I found the issue. Please make this try: go in your Steam Folder\SteamApps\common\assettocorsatracks\magione\data and open audio_sources.ini.
Change the following line:
ENABLED=1
to
ENABLED=0
Save, launch the game, take a car and select Magione. Try now if the 5.1 is working.