You can set it to 0 actually.
I found it in the Eagle Dynamic Program files folder
C:\Program Files\Eagle Dynamics\DCS World OpenBeta\Mods\aircraft\FA-18C\Options
Then open the file:
AircraftSettings.lua
and modify the section that you want. For me, it was these following sections:
{ id = 'Volume Panel', name = _('ot_Volume Panel', "Volume Panel"), type = 'section', childs =
{
{ id = 'TCN', name = _('ot_TCN', "TCN"), control = 'spinbox', defValue = 0, min = 0, max = 100},
{ id = 'AUX', name = _('ot_AUX', "AUX"), control = 'spinbox', defValue = 100, min = 0, max = 100},
{ id = 'MIDS', name = _('ot_MIDS', "MIDS"), control = 'spinbox', defValue = 100, min = 0, max = 100},
{ id = 'RWR', name = _('ot_RWR', "RWR"), control = 'spinbox', defValue = 50, min = 0, max = 100},
{ id = 'WPN', name = _('ot_WPN', "WPN"), control = 'spinbox', defValue = 50, min = 0, max = 100},
{ id = 'ICS', name = _('ot_ICS', "ICS"), control = 'spinbox', defValue = 100, min = 0, max = 100},
}
},
Just set the defValue to 0.
I haven't checked the modifications on servers but it works on single player
o7