I'm not sure I understood what you need... Try this:
WITH XMLNAMESPACES ('uri1' as media, 'uri2' as live)
SELECT RTRIM(Title) AS 'media:title'
, [Description] AS 'media:description'
, Url AS 'live:link'
, DateCreated AS 'media:pubdate'
, thumbnail AS 'media:thumbnail'
FROM dbo.Videos FOR XML PATH('item'), ROOT('root')