Here are some facts:
There are some ways in which the sound fails to reach the opponent (or miss in the game's language):
- If they are Soundproof.
- If they are immune to the move's type (like Ghost being immune to Boomburst, a Normal move).
- If the Pokemon has Wonder Guard.
- Accuracy has been lowered.
- They have a naturally low accuracy (Sing, Grass Whistle etc.)
- If the effect sof the move fails to avail against the opponent (Vital Spirit negating Sing)
Other than that well... there's only one thing about such things.
Since Pokemon move around during a battle, it's possible that at some point the opponent is so much further away from the Pokemon that the sound simply doesn't reach it's ears (or miss as you might call it).
In cases of moves like Sing and Grass Whistle, that have a naturally low accuracy, it's probably because it is not a reliable means in Pokemon.The opponent might hear it, but stays unaffected due to unreliability of the move itself.
But these are all theories, the real answers are hiding with Gamefreak (probably).
Hope I helped!