So you have a pile of nearly 300 pounds of walnuts just lying there on the concrete garage floor. You take one and test drop it from a foot above the concrete floor. Do it from different heights. About 5 to 6 inches will be the opitmal height for your testing. The solid dense walnut will not bounce and will have a definite solid sound on landing. The scaling on the inside of the shell holds the nut in a spring like suspension. It absorbs the momentum and prevents the nut from bouncing. The good nuts that are partially split will sound hollow but will not bounce.
The partially or dried out seed will bounce and have a distinct hollow sound.
Walnut trees are very suspceptable to fungi and mold. It follows for the nut too. Wet walnuts that are not dried properly develop mold and mildew inside the shell and spoil the nut.

In a small town, it is good to have an Italian who runs a roadside grocery from his vegetable farm and fruit orchard. He happens to have have a half dozen nut trees of different variety, walnut, pecan and filbert (hazel). The explanation of sorting walnuts. The sound is the best indicator of those walnuts that you could 86. A hollow sound would tell you that the nut is dry or the shell is partially split. You would end up with a good and bad pile.
A better way of sorting would be to understand the physical characteristics of the growing and maturing walnut.
So you have to go back to July when the nut has grown to near its mature size on the tree. At this time the shell has not started to form yet. Take nut and cut it longitudinally from the stem to the tip, you will see the umbilical vein from the stem into the the nut sack. It will be the only opening in the shell of the mature walnut. When the nut drops to the ground in late fall,then covered with leaves they absorb moisture. It stays dormant till the spring and the process continues. Fermentation starts to assist the nut to change chemicaly and initiate the growth of root shoot and finally primary leaf stem. In early spring if you break open one of these buried nuts you would find that the outer skin of the nut meat has darkened, the nut itself is soft and has changed to a creamy white. At this stage you can take these nuts, dry them and get a totally different tasting walnut. I don't know if anyone has done this commercially.
You wait another few weeks, test a nut and you will see that the shell has split and opened a bit. The nut has swollen and if you cut it in half the creamy white has changed to a yellowish white. It now has the staining and bitter characteristics of the walnuts root, inner bark and leaves.
Getting back to July, if you cut the walnut at 90 degrees to the stem (across the walnut) you will see the following: At the center is the very rough outline of the nut sack, similar to the outer folds of the human brain. It will form the light brown skin enclosing the meat of the nut.
Learning about the meaning of Walnut. The spellings are Middle English walnot, from Old English wealhhnutu. Maybe an ancient connection in the creation of the word. I think there are these expressions 'He's a nut case' or 'Nuts to you!'.
In the cutting, there was released a clear liquid from the nut sac and it should be hollow. The liquid inside this sack will eventually solidify in maturation to form the meat of the nut - the seed. The juices released in cutting the immature walnut (husk and core), readily stain the skin of your fingers a yellow to a very dark black brown. You should wear some type of plastic gloves, that's if you don't mind walking into your bank and having the teller remark on the color of your skin...pitch dark brown.
The portion that surrounds the nut sack is a light white green. In the maturing and drying process on the tree, this pulp forms the light brown scale on the inside of the shell. It drys away from the seed, towards the inner side of the shell but still holds the nut in a 'spring like' suspension.
Another demarcation line is just about 3/16ths to 1/4 inch from the outside of the walnut. This line will form the stone or shell of the walnut. By second week of August the shell has formed rock solid. Back in mid July, cutting a nut in different directions you would find that slivers of shell forming in a star pattern from the stem, tip, and along the ridge of the shell halves. In order to get green walnuts that would be used for pickling you would have to pick them before any shell slivers have formed. The pickling of walnuts is no different than the process used for olives. Except large amounts of water, salt, and time is expended in extracting the bitterness from the green nuts.
Finally the 3/16ths to 1/4 inch darker green portion will turn out to be the husk of the walnut. From September and into October the nut has been drying out on the tree. The husk has split and exposed the shell to the weather. Either animal, human or wind intervention knocks the nut to the ground. Crows just love to peck at the nuts in the tree to break the shell and get at the meat. They knock quite a number to the ground. Some crows have learned to take the walnut husk strands in their beaks (holds the nut in the husk) and carry the nut aloft in the sky and then drop them on the street below to break open the shell. You can prematurely knock down walnuts before the husks split at the end of September depending on climate and geographical location. When you knock them down the husks burst open on hitting the ground, releasing the nut. The ones that do not break open will do so when you step on them or roll them underneath your foot.

Regards,