Both chips allow "forced volume" which is another way of saying sample playback.
It generally takes lots of processing because not only do you have to put an amplitude value in the volume register every scanline or two but you programmatically have to deduce each sample's value, either by calculation or lookup from stored samples.
Pitfall II has the advantage of the chip on the cart which does those other things which cuts out over 90% of the work otherwise required. All the CPU needs to do is read the output register and store the sampled value.
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