You could ignore the namespace by using a pseudo-namespace and a * to name all namespaces:
;
WITH XMLNAMESPACES(DEFAULT 'some_string')
SELECT
pref.value('./text()[1]', 'varchar(MAX)') AS 'BankAccount'
FROM @MyXML.nodes('//*:GetEmployeeDetails/*:return/*:BankAccounts/child::node()') AS RequestXml(pref)
where pref.value('local-name(.)[1]', 'varchar(100)') ='BankAccount'