Trumps “admission of guilt” within that audio would have been very damaging to him at trial. If it were true. If in fact Smith had those documents that Trump was boasting about.
But because that audio was full of “bravado” Smith figured he could leak it to CNN and try to create damage to Trump in the court of public opinion.
Did it work? Do people still think Trump boasted about having classified documents? Probably.
He very well might have leaked it because he thinks it might not be allowed by the judge. Now any potential jurors have heard it.