Try This

DECLARE @x XML
SELECT @x = '
<AuditTrail xmlns="http://AuditSchema"> 
  <Table TableName="tblClient"> 
    <Field xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" FieldName="ClientID"> 
      <Before>2919529</Before> 
      <After>2919529</After> 
    </Field> 
        <Field xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" FieldName="DateAltered"> 
      <Before>2008-02-02T13:37:32.483</Before> 
      <After>2008-02-02T13:37:32.983</After> 
    </Field> 
    <Field xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" FieldName="tempClientID"> 
      <Before xsi:nil="true" /> 
      <After xsi:nil="true" /> 
    </Field> 
  </Table> 
</AuditTrail>'

;WITH XMLNAMESPACES (
    default 'http://AuditSchema'
)
SELECT
    x.value('Before[1]','VARCHAR(30)') AS Before,
    x.value('After[1]','VARCHAR(30)') AS After
FROM @x.nodes('/AuditTrail/Table/Field[@FieldName="DateAltered"]') a(x)

/*
Before                         After
------------------------------ ------------------------------
2008-02-02T13:37:32.483        2008-02-02T13:37:32.983
*/