What my players hear depends entirely on how well they roll. Nat 20? What do you know, the orcs just happen to be going over their plans. Nat 1? The orcs hear you. A passing roll, but just barely? You overhear one of them fart and 2 others laughing hysterically.

This is a situation with an element of luck and you’re literally using luck to determine if they even succeed; why not incorporate the numbers in the roll to reflect the random nature of the game world’s reality?