Welcome here beavisbeaver
Many thanks for your advices. I got a spare sound board... unknown condition but I really hope some parts are still working and may help to fix mine.
The sounds you are probably missing are the tire screech & crash / explosion sound. They seem to be separate from the other engine, overtaking cars & tunnel sounds which come thru the L & R channels.
Nope, I can hear these sounds (check the video I've done)... I can't really tell what's missing because everything seems there. My guess it was a duplicate sounds channel that goes to the rear/seat I don't have...