Sathya Sai Baba's Discourse on Krishna's Birthday, 31 August 1983, in the Purnachandra. Above is Part 1; below is Part 2. We have broken it into two parts because of its length (2 hours).
In Sathya Sai Speaks volume 16, it is titled "Why the Avatar comes". Here is it is: pdf.
But the paper version takes only three pages, and it is basically only a summary of this 2-hour discourse.