This makes a big difference
The file audio_engine.ini is located in: ...\steam\steamapps\common\assettocorsa\system\cfg
Change the value of max channels to 512 from 256. It should then look like this:
[SETTINGS]
ENABLE_AUDIO=1
MAX_CHANNELS=512
UPDATE_INTERVAL=0
ENABLE_PERFORMANCE_SPEW=0
LIVE_UPDATE=0