In general the interior sound depends on the truck model.
You can drive with the 'Promods' map though, where you have to choose the level of interior cockpit sounds when downloding the first general file. Afterwards the 2 leftover files are standard. AND it's a beautiful map! http://www.promods.net/
Otherwise the interior sounds are very low for the Volvo FH. Renault, MAN, old DAF are a bit louder.