The Mouth of Sauron

[Balin999]

The Seven are accounted for, the Nine Sauron holds...

I think that with this it is meant that Sauron had the control over the Nine via the Nazgul. I don't think that he had taken back their rings. Without them, he wouldn't have an instrument to control their spirit. But then, how could his armies falter after he bent his mind on getting his one ring? That doesn't make sense.
As for the Mouth having a ring himself, I think we have agreed earlier that he had no ring for himself.