The bug

The muffling effect that's supposed to happen to the rain sound effects when the rain is falling on a block above isn't determined by head position, i.e. when swimming or wading (on a non-full block like a slab or chest) in water during a rain storm with the player's head above the surface of the water the rain will sound as if it's hitting a ceiling above even thought it's falling right next to the player's head.
I can also sometimes get this same effect when having most of the player's body covered in solid blocks and just the player's head above, but not consistently.