What I expected to happen was...
When two people stand on the same block and stand still, no noise should come from the players.

What actually happened was...
When two people stand on the same block, you can hear the noise that would be made by walking over that block, be it sand, stone, wood, etc.

Steps to Reproduce
1. Open a world to LAN, survival, no cheats, hard mode.
2. Have another player join, and put both players on the same block.
3. Turn your volume up if necessary.
4. You should hear the sound of a player walking over the type of block you are standing on. I have made this work with Sand, Polished Diorite, Polished Granite, Acacia Wood Planks, and Grass.