Well thanks, but that mod is useless now. The locations changed in the last DCS version. In the meantime I have found it myself. The loud engine noises of the P-51 are in:
\DCS World\Sounds\Effects\Aircrafts\Engines\V_1650_7\Cockpit
I have found out it deleting temporarily all the files inside, and the cockpit turned silent,
then restored the files, and lowered the volume.
VERY ODDLY enough the Spitfire files are not in \Sounds but in :
\DCS World\Mods\aircraft\SpitfireLFMkIX\Sounds\Effects\Aircrafts\SpitfireLFMkIX\Cockpit
Here we have it again: the folder structure of DCS is utterly absurd.
I remember another SIM (I do not mention the name otherwise this post will be deleted), in wich the folder structure was \Aircrafts\Planexxx\Sounds\Textures\Model\etc.
That made very easy to apply mods, textures or update.
No wonder it takes so long for ED to update and / or change things.......