This is a redo of #436.
I've taken a single, longer audio sample and extracted 3 samples
of 2.0 length, and gave them all a 0.5 sec fade-in and fade-out.
I added a 4th sample that's essentially a nearly-quiet version of
one of the prior sounds.
This combined with the rate of footstep sounds results in a
randomly changing underwater sound that blends in and out somewhat
nicely.
Code entirely by @MirceaKitsune.
