Freshly fallen snow is not a good reflector of sound waves. The snow is filled with many small gaps that work to absorb sound waves. Waves that are absorbed do not reach your ears. If more waves are absorbed, your ears will detect fewer waves, thus leading you to hear less than a normal day. Once the snow has a chance to melt and/or harden, it becomes more reflective and waves reach your ear as normal.
Check it out the next time it snows and ask your kids what they hear and how things are different than before it snowed.