They spend a lot of time and resources collecting debris and looking for black boxes to find the root cause. Every crash is different, and SONAR has limitations. I don’t know what kind of signal is emitted by the boxes. I thought it was a high frequency ping. Maybe a different kind of signal that penetrates water and earth, and can provide directional homing.
MS804 didn't crash in the Soviet's controlled water and even KL007 black box was recovered after the cold war. I'm positive they'll find the MS804 black box.