Function that takes a sample chunk and processes the chunk. It should
also return a sample chunk containing the processed data. If wished, the
chunk may be side-affected and the input chunk returned. It should not be
assumed that the input chunk will be side-affected, but it must be noted
that it is possible that it could be. This process function may also
return null. If null is returned it means that the rest of the audio
stream is not required to be processed by this processing function.
Whether the rest of the sample chunks are copied or ignored is up to the