Back on this
Eventually found an other PS where the 12V is working.
...but still some sounds missing!
So after a close investigation I've discovered that the sounds (4 channels) provided by the PCB seems ok.
I've used a regular home computer amplifier/speakers to listen what coming for the 8 grey wires going to that connector...
...and It gives this :
1-2 : 'in game' sound effects (those who are missing!
[channel 1]
3-4 : Background motor sound (only when I drive inside tunnel) [channel 2]
5-6 : main motor sound [channel 3]
7-8 : main motor sound (I didn't spot any difference than the previous channel) [channel 4]
Sooo... I obviously have a problem with the amplification of the first channel (1-2 pin).
Sooooo... while I'm waiting an other spare sound board to arrive, I've just 'shifted' the connector (the one in the orange spot) to the left... so I've lost one of the four channel, the channel 4 which provides obviously the same signal than the channel 3...
In game, I do not spot any sounds missing, check this out
Remember, a seat (which I do not have like other BB owners) is supposed to be connected to that cabinet, and there's is a connector going from the cabinet to it. My guess is that extra channel is for the speakers on that seat. This is something I'm going to check.
But by now, I can *at last* enjoy some Buggy Boy games!