Hi there!
(Because this answer was posted before, I will be posting my previous response to the question here.)
The courts decided in United States vs. Nixon that the doctrine of separation of powers nor the need for confidentiality of high-level communications granted by the executive privilege warranted absolute presidential privilege. In other words, it upholds that the president does have executive privilege but it is not an absolute privilege. When it comes to the executive branch and even the president, due process of law still applies. This reinforced that even the president could be prosecuted under the due process of law and as a result, was forced to release the tapes and documents requested.