Kelp, coral sounds: Add missing 'footstep'. Use quiet 'dig', 'dug'

Footstep sounds as sand or cubic coral nodes.
Dig and dug sounds as leaves but much lower gain to be suitable for
underwater.

mods/default/nodes.lua

@@ -1929,6 +1929,10 @@ minetest.register_node("default:sand_with_kelp", {
 	},
 	node_dig_prediction = "default:sand",
 	node_placement_prediction = "",
+	sounds = default.node_sound_sand_defaults({
+		dig = {name = "default_dig_snappy", gain = 0.2},
+		dug = {name = "default_grass_footstep", gain = 0.25},
+	}),
 
 	on_place = function(itemstack, placer, pointed_thing)
 		-- Call on_rightclick if the pointed node defines it
@@ -2000,6 +2004,11 @@ minetest.register_node("default:coral_green", {
 	},
 	node_dig_prediction = "default:coral_skeleton",
 	node_placement_prediction = "",
+	sounds = default.node_sound_stone_defaults({
+		dig = {name = "default_dig_snappy", gain = 0.2},
+		dug = {name = "default_grass_footstep", gain = 0.25},
+	}),
+
 	on_place = function(itemstack, placer, pointed_thing)
 		if pointed_thing.type ~= "node" or not placer then
 			return itemstack
@@ -2028,6 +2037,7 @@ minetest.register_node("default:coral_green", {
 
 		return itemstack
 	end,
+
 	after_destruct  = function(pos, oldnode)
 		minetest.set_node(pos, {name = "default:coral_skeleton"})
 	end,
@@ -2051,6 +2061,11 @@ minetest.register_node("default:coral_pink", {
 	},
 	node_dig_prediction = "default:coral_skeleton",
 	node_placement_prediction = "",
+	sounds = default.node_sound_stone_defaults({
+		dig = {name = "default_dig_snappy", gain = 0.2},
+		dug = {name = "default_grass_footstep", gain = 0.25},
+	}),
+
 	on_place = function(itemstack, placer, pointed_thing)
 		if pointed_thing.type ~= "node" or not placer then
 			return itemstack
@@ -2079,6 +2094,7 @@ minetest.register_node("default:coral_pink", {
 
 		return itemstack
 	end,
+
 	after_destruct  = function(pos, oldnode)
 		minetest.set_node(pos, {name = "default:coral_skeleton"})
 	end,
@@ -2102,6 +2118,11 @@ minetest.register_node("default:coral_cyan", {
 	},
 	node_dig_prediction = "default:coral_skeleton",
 	node_placement_prediction = "",
+	sounds = default.node_sound_stone_defaults({
+		dig = {name = "default_dig_snappy", gain = 0.2},
+		dug = {name = "default_grass_footstep", gain = 0.25},
+	}),
+
 	on_place = function(itemstack, placer, pointed_thing)
 		if pointed_thing.type ~= "node" or not placer then
 			return itemstack
@@ -2130,6 +2151,7 @@ minetest.register_node("default:coral_cyan", {
 
 		return itemstack
 	end,
+
 	after_destruct  = function(pos, oldnode)
 		minetest.set_node(pos, {name = "default:coral_skeleton"})
 	end,