Wow........that is the most long winded and confusing half explanation of "pound sound" I can imagine. They try, in a detailed manner, but still not sure I completely understand. For one the phrase seems to be used in different ways by different people, from what I am reading :
One being "pound sound up your..." rear or ear, however vulgar you want to be, which is a phrasing I never heard, just seems to be a reference to tell someone off and to do something unpleasant. This seems more in line with how I heart it used, just never heard the full phrasing like that
The other seems to be doing something pointless, because "pounding sound" is pointless. Yet its not always pointless, that is the problem with that interpretation. You can pound sand into different types of holes, as mentioned, and you can pound sound to condense it and make it more dense, that is basically how sound castles and structures are made, to some extent, especially if its wet, so its not always pointless.