That markup is not well-formed, you need to bind the prefix 'media' to a
URL.
Here is an example how to write XML with XmlWriter:

        Const ns As String = "http://example.com/2008/media"
        Dim settings As XmlWriterSettings = New XmlWriterSettings()
        settings.Indent = True
        Using writer As XmlWriter =
XmlWriter.Create("..\..\XMLOutput1.xml", settings)
            writer.WriteStartElement("item")
            writer.WriteElementString("title", "Sunshine up Ahead")
            writer.WriteStartElement("media", "credit", ns)
            writer.WriteAttributeString("role", "author")
            writer.WriteString("Peter Jones")
            writer.WriteEndElement()
            writer.WriteStartElement("media", "content", ns)
            writer.WriteAttributeString("url",
"http://www.jeroenwijering.com/upload/peterjones_sunshine_lofi.mp3")
            writer.WriteAttributeString("type", "audio/mpeg")
            writer.WriteEndElement()
            writer.WriteEndElement()
        End Using

Result looks like this:

<?xml version="1.0" encoding="utf-8"?>
<item>
  <title>Sunshine up Ahead</title>
  <media:credit role="author"
xmlns:media="http://example.com/2008/media">Peter Jones</media:credit>
  <media:content
url="http://www.jeroenwijering.com/upload/peterjones_sunshine_lofi.mp3"
type="audio/mpeg" xmlns:media="http://example.com/2008/media" />
</item>

If you want to avoid the duplicated namespace declaration then write out
the namespace declaration explicitly as in

        Const ns As String = "http://example.com/2008/media"
        Dim settings As XmlWriterSettings = New XmlWriterSettings()
        settings.Indent = True
        Using writer As XmlWriter =
XmlWriter.Create("..\..\XMLOutput1.xml", settings)
            writer.WriteStartElement("item")
            writer.WriteAttributeString("xmlns", "media",
"http://www.w3.org/2000/xmlns/", ns)
            writer.WriteElementString("title", "Sunshine up Ahead")
            writer.WriteStartElement("media", "credit", ns)
            writer.WriteAttributeString("role", "author")
            writer.WriteString("Peter Jones")
            writer.WriteEndElement()
            writer.WriteStartElement("media", "content", ns)
            writer.WriteAttributeString("url",
"http://www.jeroenwijering.com/upload/peterjones_sunshine_lofi.mp3")
            writer.WriteAttributeString("type", "audio/mpeg")
            writer.WriteEndElement()
            writer.WriteEndElement()
        End Using

That way the result is

<item xmlns:media="http://example.com/2008/media">
  <title>Sunshine up Ahead</title>
  <media:credit role="author">Peter Jones</media:credit>
  <media:content
url="http://www.jeroenwijering.com/upload/peterjones_sunshine_lofi.mp3"
type="audio/mpeg" />
</item>