To gett total Tax amount of sales order in axapta you can try below code in job .
SalesTotals salesTotals;
SalesTotals_Trans salesTotals1;
SalesTable salesTable;
SalesLine salesLine;
TaxAmountCur Amount,amt1;
Tax tax;
;
salesTable = SalesTable::find("S0000343_052");
salesLine = SalesLine::find(salesTable.SalesId);
salesTotals = SalesTotals::construct(salesTable);
tax = Tax::construct(NoYes::No);
Amount = salesTotals.totalTaxAmount();
info(Strfmt("The tax amount is %1",Amount ));