If the frequency of a sound decreases, what happens to the wavelength

The product of (frequency) times (wavelength) is always the same number ...
the speed of the wave. 

So if one of them changes, then the other one must change by the same
factor in the opposite direction, in order for the product to not change.

This works for sound and all electromagnetic waves.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.