Actually, the waves are fully formed right at the speaker (electro-mechanical to acoustic energy transfer). For a sound wave to gain amplitude (i.e., fully form) in free space, it would need some sort of energy input.
Sort of like when a bullet exits the barrel of a gun, it's going full velocity at that point and it won't speed up in the air on its own accord.
However, like a bass wave, a bullet can ricochet around and do all manner of odd things. But it certainly won't be going any faster.