Called just before simsound loops through channels and propagation paths
so that parameters of a new sound can be set up. Generally, if the sounds
are deterministic and all the same, nothing has to be done here. but if
sounds contain random elements, then it will be necessary to set up the
random bit here so that the generatet sound is the same on all channels.
sampleOffset - sample offset - 0<=sampleOffset<1 How far into the sound the first sample is, i.e.
0 means that the first sample point will be right on the start of the sound (value 0
if it's sin wave based), value 0.5 would be half a sample later.