Did Sauron ever find out that Isengard was attacked?

Question

Did Sauron ever find out that Isengard was attacked? Saruman would probably be reluctant to tell him. Sauron was probably watching more important things at the time, like the Battle of Helm's Deep, preparing the attack on Minas Tirith, and later the battle at the Black Gate. Did Sauron find out that Isengard had been defeated before the ring was destroyed?

Answer 1

Yes. At the Black Gate (The Black Gate Opens, the Return of the King), the Mouth of Sauron says

West of the Anduin as far as the Misty Mountains and the Gap of Rohan shall be tributary to Mordor, and men there shall bear no weapons, but shall have leave to govern their own affairs. But they shall help to rebuild Isengard which they have wantonly destroyed...

Answer 2

Yes.

In The Two Towers, Book 3 Chaper 11 (The Palantir) a Nazgul travels to Isengard shortly after Pippin looks into the Palantir:

'Nazgul!' he cried. 'The messenger of Mordor. The storm is coming. The Nazgul have crossed the River! Ride, ride! Wait not for the dawn! Let not the swift wait for the slow! Ride!'

Later in the same chapter Gandalf confirms the purpose of this messenger:

A messenger has been sent to find out what he is doing. And after what has happened tonight another will come, I think, and swiftly. So Saruman will come to the last pinch of the vice that he has put his hand in. He has no captive to send. He has no Stone to see with, and cannot answer the summons. Sauron will only believe that he is withholding the captive and refusing to use the Stone. It will not help Saruman to tell the truth to the messenger. For Isengard may be ruined, yet he is still safe in Orthanc.

He also speculates some on other information that the Nazgul might learn:

It may be that the counsels of the Enemy will be confused, or hindered by his wrath with Saruman. It may be that he will learn that I was there and stood upon the stairs of Orthanc – with hobbits at my tail. Or that an heir of Elendil lives and stood beside me.

So Sauron certainly knew that Isengard had been destroyed at some point during or, at latest, shortly after Gandalf's and Pippin's ride to Minas Tirith, and may have even had more information.

Answer 3

Gandalf believed Sauron did not immediately learn of the fall of Isengard.

In the books, after Pippin looked into the Palantir which Grima Wormtongue had thrown down from Orthanc, Gandalf surmised that Sauron believed Pippin had been taken prisoner by Saruman and made to look into the Palantir as an act of cruelty.

From this, we can guess that Sauron was unable, or unwilling, to simply look through his own Palantir and see what had happened to Isengard.

The question was not explicitly raised again -- the main characters were focused on more urgent matters, just as Sauron was. (Edit: This is incorrect, as noted in other answers -- it was discussed, and Sauron sent a Nazgul to Isengard to investigate.)

However, it was not in Sauron's nature to trust Saruman. He almost certainly had spies in Isengard. One of these would eventually report back. It might only take a day or two if the agent had a fast way of sending messages (through birds like Saruman's crows, or by making contact with one of the Nazgul). On the other hand, travelling overland from Isengard to Mordor would have taken some weeks, especially since the agent would have to be wary of patrols from Rohan or Gondor.

So Sauron probably learned of the fall of Isengard before the destruction of the Ring, but it is plausible that word might not have reached him in time.